The term jam in poker is used to describe an action in which the player bets their entire stack, i.e., moves all in. The term is quite popular among the younger generation of players.
For example, you could hear a player saying: I opened from the button, and the big blind jammed on me. This means that the player in the big blind moved all-in after the BTN raise.
The poker term jam comes in a few different variations that you may encounter. Open jamming refers to a player moving all in when it’s their turn to act, and there is no raise in front of them. When a player moves all in after a player before them has raised, this is known as jamming over the top.
Although this is a bit of a slang term, it’s been adopted quite widely by the poker community, so you may hear it from commentators on your favorite poker show as well.