The limit on raises per round, or on the amount bet per hand.
In Limit games, the default is 4 bets per round. Example. Once a bet and 3 raises are in, the round cannot be raised any further. Many rooms uncap it when only 2 players are in at the start of the round, and a Stud completion does not count as a raise. Even in no-limit there are "cap games" with a per-hand maximum, and "capped" in range talk means something else.