What's an RPG?
Aside from a rocket propelled grenade? It's a role-playing game!
And maybe at that point you're like, "Okay, what's that?"
The short version is "playing pretend" and the only slightly longer version is "playing pretend... with dice!"
Anyone who ever played cops and robbers or similar games of imagination as a child might remember just how fun all that was and may also remember at least one occasion of the "I got you!" "Nuh uh!" argument. The good news is that playing pretend is just as fun at any age and the aforementioned dice and accompanying rules eliminate all doubt as to who did, indeed, get whom.
This is a role-playing game, or RPG for short. Occasionally you may see this referred to as arpee as the phonetic expression of the first two letters of the abbreviation.
These games, like lots of niche pastimes, come with lots of jargon that may not be necessarily familiar to the uninitiated or might use common terms in unusual ways. That might cause some confusion and act as a barrier to entry. This isn't intended as gatekeeping. Just like the terminology of other hobbies (and professions), the purpose is to communicate complex ideas in a short amount of time. For instance, XP is familiar to lots of both tabletop and video game players as expanding to eXperience Points. To help clarify some of this, I present the following glossary:
Common to many role-playing games, these are generally expressed by a numerical value and describe the innate prowess of a character. Often these will describe things like mental or physical strength and fortitude and sometimes the attractiveness or repulsiveness of a character's appearance or personality.
The depth and complexity of a game's rules.
In many RPG's this is the real goal of all of a character's many tasks. Returning with the stolen idol, defeating the villains, saving the city or slaying monsters may result in loot (see below) but the best reward is the gain of enough XP to level up (see below) and become more powerful so your character can tackle greater challenges and fell mightier adversaries.
By any other name, the extra details remain the same. These are usually character details that generally confer some form of advantage to the character or even the whole team. In some RPG systems (see below) these extras may be conferred by purchasing them with points or, as in Fate, could simply be narrated into existence or alternately purchased if the extra is especially potent.
Combat or other physical threats often make up the core gameplay of many RPGs and, as such, the ability to keep track of your character's physical well-being becomes important. This might come in the form of Hit Points as in a certain popular RPG involving scaly fire breathers and secure underground lairs, or in the idea of health levels that diminish ability as wounds stack up. In this game, health is handled by Stress and Consequences, instead, as it is intended more for creating compelling fiction and less so for either gritty realism nor as a strategic war game with a storyline.
A character's (or NPCs, below) level is a generally numerical expression of the relative power of that individual. This is most commonly found in games that center on combat as a primary means of character growth, both on the table and in video games. There are commonly two methods of leveling up in narrative games: accruing a target amount of XP (above) and Milestones (below). XP targets generally increase in size with each level so that one must slay more enemies, or more powerful enemies, with each level increase to maintain a good character progression rate.
The other primary reward of many combat-centric games, it is also often a primary way to increase a character's potency as important as increases in attributes and skills (below) or even moreso. In sword and sorcery titles some enemies may be immune to mundane weapons, demanding the PC's (below) locate magical weapons to take on the threat. This can often be a quest in itself.
In games with more crunch (above), there may be significant bookkeeping involved and one of those matters may be how often certain abilities may be used. This might be expressed by a "mana pool" representing the magic reserves available for casting spells with variable costs or in different sorts of timers such as once per game day or once per fight.
This is the part of the game that uses dice. These systems may be heavier or crunchier (above) that involve at least arithmetic of fair complexity, or they can be rules light to the point of practical non-existence. This latter might be called "freeform role-playing games" and can closely resemble improv theater or collaborative fiction. The crunchier games like the most well known are descended from tabletop strategy wargames and that heritage is usually apparent. Fate is much more rules light with Fate Accelerated even more so.
Narrator/Game Master (GM)/Dungeon Master (DM)
Everyone involved in the game may be playing, but in many RPG's one player's role is different from everyone else's. By any of these names, and many more, this is the player that "runs the game". Usually the most experienced, anyone at the table can run a game. All that is required is a familiarity with the rules and some rough ideas for a storyline that you can freely lift from any source of your liking. If you like wizards, zombies or superheroes, the potential for storylines is a streaming service away.
This is everything in the game world that is specifically not controlled by one of players (as opposed to the Narrator, above). If it is remotely a character and has any sort of agency at all, from the hapless victim to the dastardly do-badder to the innocent bystanders to the police on their way to the scene, they are an NPC. In Fate there are differences between most people in the world (nameless NPCs, people important to the story (Named NPCs), and the protagonists of the story: the Player Characters (below). All of these people and things are portrayed by the Narrator, though other players might help out in some scenes by picking up a side character while their own charactees are "off screen".
The protagonists of the story, likely to be heroic (but not necessarily). These are the main characters of your movie that plays in the theater of the mind's eye or maybe on an online digital battle map. Everyone in the game may be playing, but one of them is not a Player of one Character. That person is the Narrator (above). Player Character's or PC's are usually the creations of the individuals at the table and are the personas they adopt to invoke change upon the fictional world they are all building together.
These describe the things a character learns to do rather than their physical prowess. Skills may describe a very wide range of abilities from medicine to murder and every other practiced task a person might undertake. These often have a numerical value that describes the relative mastery the character has performing said task from apprentice to master.
There are more (lots more) examples of this sort of jargon that are specific to individual games or might express many of these same ideas but by a different name, as seen above. The Fate System has quite a few examples of its own: Approaches, Aspects, Fate Points and more.
If all of this makes you hungry for more, you should peruse the rest of the site. For those interested in playing the game (as opposed to running it) there is a section on character creation. For the would-be Narrators, there is a section for you, too.