Swapping a card for another before the start of a round.
After the initial hand is drawn from the deck, a mulligan gives the player the opportunity to swap a card for another in their deck before play begins. In Gwent, each player is given two mulligans per round, with the player who goes first getting an additional mulligan in the first round.
In the second and third round, if the player draws more cards than is allowed by the hand limit, the additional cards are not drawn and instead, for each card not drawn, the player receives an additional mulligan. For example, if a player has 8 cards in hand at the end of round one, they will only draw two cards instead of three and receive one additional mulligan for a total of three mulligans at the beginning of round two.