We've all learned about "counting outs" in hold'em, an early exercise in estimating hand strength before all of the community cards have appeared.
Say the board reads and you hold . Your hand likely isn't best at the moment, but with eight "outs" to make a straight (the 4 sevens and 4 deuces) you can calculate your chance of improving to the best hand, thereby helping you decide whether or not to continue.
In that example your outs are all "clean" — that is, all eight would improve you to an unbeatable straight. But say the board was instead, meaning a third heart could possibly fill an opponent's flush.