An out is a card that can significantly improve the player’s hand. This term is especially used when the player’s hand is an underdog.
For example, let’s say that the community board reads 10h 9s 8h 4c, and you have an Ah Kh hand while your opponent has a Jc 7s hand. As your opponent already has a straight, you’re the underdog in this situation. You can win on the river only by getting one of the remaining nine hearts cards that would help you make a flush. In other words, you have eight outs.