Calling means matching the amount already bet in the current round so that you stay in the hand. If the player before you bets $50, you call by putting $50 into the pot; the action then moves to the next player, usually clockwise.
It is one of a short list of choices on each turn. With no bet to face, you can check or bet; facing a bet, you can call, raise or fold, and in no-limit games you can also move all-in. A player who calls an all-in with fewer chips only matches what they can afford.
Many online poker rooms let players pre-select a call before their turn, which keeps fast games moving. The choice is usually locked once the action reaches you, so it is best used when you already know you will continue. For operators, clear action buttons and time banks help prevent misclicks and disputes at the virtual table.