Definition of out
An out is a card that can save you, by improving your hand. For example, you are all in with JQ in spades on 9-10-2 with two spades. Your opponent has AT for top pair top kicker. Here you have several outs (cards that will give you the best hand). All 8s are outs, all kings, all jacks, all queens and all spades! With this hand, even if you don't have a pair, you dominate AT, you are ahead in equity.
Calculate your equity based on outs
Here's a simple trick to approximate your equity in a situation, based on your number of outs. When you have found your number of outs, multiply that number by 4 if you are on the flop and by 2 if you are on the turn. For example, you have a flush draw on the flop against an overpair. A flush draw will give you 9 outs on the flop. So, 9 outs x 4 (since we're on the flop) = 36% Roughly speaking, you'll hit your draw 36% of the time. The number is not exact, but it is very close. If you were on the turn, 9 outs x 2 = 18%.