Jit is a slang term that refers to a person, and sometimes to a friend. The exact origins of the term aren't clear, but it's now a popular way for people to refer to one another. It's often used in a disapproving context, as it is in the phrase "jit trippin'," but it can really used in any case when you might refer a person.

There aren't many more rules than that, but you might want to hear or read other people use it a few times before you try doing so yourself.