In this usage, Kind is synonymous with species and, more specifically, an intelligent species such as Humankind or Elvenkind. All characters, whether player controlled or non-player characters (NPCs), have a Kind. A Kind in Super:Natural denotes a sapient creature capable of reasoning. Some examples would be humans, self-aware artificial intelligences and dragons. Not all Kinds are necessarily intended for player characters as portraying an ancient dragon among a group of hobbits would be a bit unbalancing. Of course, it's your game, so you do what you want.
Each Kind in Super:Natural has Kindred Features that denote physiological function such as a natural ability to heal at an accelerated rate (Regeneration) or the ability to see in the dark. Kindred Features may also be negative, such a particular sensitivity to sunlight or a weakness to fire. Kindred Features are built in Advantages, Disadvantages, Powers and Weaknesses that are simply part of life as a member of a given Kind. In many cases the benefits balance out with the drawbacks, such as a Kind that can regenerate but is weak to fire. Sometimes it may seem that there is no downside or, alternatively, that there is no advantage to playing as a human. That is up for you to decide.
Each Kind in Super:Natural also has Kindred Standards. These are oversimplifications of common features many, but not all, members of a given Kind tend to exhibit. Kindred Standards are intended as a guide for players new to role-playing games or who prefer following convention. These are intentional stereotypes of a given people that might simplify character creation.
It is easy to say, "Oh, those elves... mages and rangers. All the bows and the spells." It may be true that an individual Valori might make for a great wizard or skilled ranger, but it might be equally true that a given Valori could be a talented chef, a singer with an angelic voice, or a backstabbing assassin. Any member of any Kind is limited only by the availability of training and the technology level of their current planet.
Kind is only a statement of physiology but does not speak to potential. A being of any Kind can have any Job.