In essence, a jam is an organized musical space within which one can exercise one’s chops. There are certainly some rules of engagement that might vary from jam to jam, that I don’t know as I’m not a player, but there is the all important vibe. It’s certainly not a beginner’s training session, but it isn’t a clique, either.
It’s all about a supportive atmosphere, with jazz as an integrity that maintains each jam via a technique borrowed from modern parenting: tough love.
In a medium that depends on split-second thinking the jam sessions are vital mental and tactical nourishments for players. And, if you love a good head-cutting session than that’s the place to check out.
Jam sessions became an art form during the golden era of jazz – 1935 through the early 1960s – when Parker, Dizzy Gillespie, Monk, Miles, and Coltrane were still new. New York was ground zero for jazz and jamming back then; in many ways it still is. Musicians often don't call out tunes. They just start playing and the others catch on, or don't.
You can meet new people, see old friends, hear new music, play great music and gain plenty of valuable experience. It is a great thing to do for an evening, if you want a break from your usual routine of chilling out at home, watching a sports game on the television or something alike.
So, go to the club, buy a drink and relax.