An out is an unseen card that will complete a player’s drawing hand or make the hand stronger. For example if you hold A♠ K♠ and the board reads 3♠ 7♠ 8♦ 2♥, your outs are any ace or king (making top pair), or any spade (making a flush).
Outs are used in determining a hand’s equity at any time during play. Let’s say that in the example above, your opponent holds a three in their hand, making a pair. You can calculate your chance of winning the hand by counting your outs. In this instance, you can hit 3 aces, 3 kings, or 10 spades for a total of 16 outs. This comes out to roughly a 34% chance that you will win, since you have 16 outs/46 possible cards.