Outs
Outs are the unseen cards that would complete your hand and likely make it the winner; the more outs you have, the better your chance of improving.
Outs are the specific cards that, if one of them shows up on the next community card, turn your hand into a strong one (usually the best hand). You count them by looking at the cards you can see, your two hole cards and the shared board, and asking which of the remaining unseen cards would help you. The more of these helping cards there are, the better your chances of improving.
This comes up most when you have a drawing hand on the flop or turn, meaning your hand isn't made yet but could get there. You count your outs, then estimate how often you'll hit. A quick beginner shortcut is the 'rule of 2 and 4': multiply your outs by 2 for one card to come, or by 4 with two cards still to come, to get a rough percent chance of improving.
Knowing your out count is the foundation for smarter decisions, because it feeds straight into ideas like pot odds and equity that tell you whether calling a bet is worth it. Just be careful not to count outs that could give an opponent an even better hand, and remember that hitting your card isn't guaranteed, it just shifts the odds in your favor. With practice chips you can count outs every hand until it becomes second nature.
Example
You hold 9 of hearts and 8 of hearts, and the flop is K of hearts, 4 of hearts, 2 of spades, giving you four hearts and a flush draw; there are 13 hearts in a deck and you can see 4, so you have 9 outs to make the flush. By the rule of 2 and 4, that's roughly 36 percent to hit by the river.