The cap poker term is very diverse, as it can mean a multitude of different things. Primarily, it refers to putting in the maximum number of raises in one betting round.
The second most common definition of the cap is that it’s the act of putting a chip on top of the hole cards for protection, i.e. so that they don’t get mucked by accident.
Additionally, it can also refer to the maximum buy-in limit for a cash game. When there’s no cap in a cash game, it means that there’s no maximum buy-in amount.