A jam session is an open sit-in: if you play an instrument or sing, you can sign up and improvise with others. There is no fixed band set, just changing line-ups through the night.
There is usually a house band or at least a backline (drums, amps, often a keyboard). You bring your own instrument – guitar, bass, horns, voice – and put your name on a list.
When it is your turn, you call the tune, key and tempo. Jazz, blues, soul or rock standards are often known; for unfamiliar songs you sketch the form. After a tune or two the line-up changes.
Listening without playing is normal and welcome.
Jam sessions are for people who want to play with others. Beginners can listen or sit in on simple blues forms; more experienced players take solos. It is not a slot for your own song set.
If you want to perform your own material, an open mic is a better fit. The difference is spelled out in Open mic vs. jam session.
Karaoke is different: you sing to a backing track with lyrics on a screen, without a band. See What is karaoke?.
OpenStage lists open sessions by city and type. Under Find events pick a city and filter by jam session.