In a 2-player match, you’ll be able to play a meld if you win a turn during the round (since 2-player Pinochle doesn't have a bidding round).
During a 3- and 4-player game, you’ll count points from all of your melds before you play the cards in your hand-this informs how high your bid can be in the bidding portion of the game. Melds are specific combinations of cards that earn points if you have them in your hand.