In all forms of poker, the term “call” is used as a verbal expression to announce your intention to match the bet(s) in front of you.
So, for example, a player from UTG raises to $15 before the flop. It folds to you on the button, and you announce a call. This means that you’ll match the $15 bet and pass the action on to the next player.
If a UTG player raises to $15 and someone re-raises to $45, if you announce a call, that means that you’re matching the $45 raise, as this is the effective bet.
Verbally declaring a call can sometimes be easier than doing it with your chips, especially when facing bigger bets. It helps avoid any confusion, as once you announce a call, you can take your time putting the chips in the pot, and there is no way for anyone to misinterpret your intentions.