What exactly is a Json Web Token? I like to describe them as an easy way to digitally sign some json data about a user, that can then be used as an access token for some kind of api. Check out JSON Web Token on wikepedia for an exact description.
I really like JWTs because they are verifiable without an additional call to some identity server. They are short-lived by default (at least should be). And it is the best option for access tokens at the moment.