Outs are the cards left in the deck that improve your hand to something that can win (or continue profitably).
Outs are used to estimate how often you will improve on the turn and river.
You have a flush draw on the flop (two cards of the same suit in your hand and two on the board).
There are usually 9 outs to complete the flush (13 cards in a suit minus the 4 you can see).