Jazz means improvising. Jazz is a universal language. A bunch of people who have never met before getting in a room and playing together. Its when the magic happens. They have to listen, they have to find the tunes they have in common. The one who calls the tune has to trust that the changes and the rhythm are gonna sit there for them. They start the tune and it all feels a bit different so they adapt and respond and change. There is a basic etiquette and they all know how it goes. That gives the safety of the platform for creativity.
A jazz jam session is where beginners can rub shoulders with seasoned professionals. Come with a jazz tune you know well enough to play the tune and a chorus of solo. Bring a chord chart for the bass player and drummer. Be prepared to count it in. Thats it! Play the tune, play your solo then step back and let the band take over. Any other lead instruments will also solo as will the pianist and the bass player. The band may even trade fours with the drummer. Then get ready to come back in with the tune. You can even have a way to end it which you may be able to telegraph to the drummer but remember – this is not an arrangement, its a jam session. Anything can happen, so be prepared to throw all your plans out the window and just listen and play.