Definition of out
A "out" (which could perhaps be translated as "way out") is a card that can save us, improving our hand. For example, you are all in with JQ on 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! Have this hand, even if you don't have a pair, you dominate AT, you are ahead in fairness.
Calculate your equity based on the outs
Here's a simple trick to approximate your fairness 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 at flop and by 2 if you are at the turn. For example, you have a color print at flop versus a overpaired. A color print will give you 9 outs at flop. So, 9 outs x 4 (since you're on flop) = 36% Roughly speaking, you will touch your print 36% of the time. The figure is not exact, but very close to reality. If you were on the turn, 9 outs x 2 = 18%.