You are dealt A-J spades on the big blind. Someone raises, and you call. The flop comes Qh-10d-9s. What do you do? In an ideal world, the turn card would be a King, giving you the nut-straight. However, an 8 also gives you a straight. How do you figure out the strength of your hand?
The answer is by determining the odds of hitting your ideal cards, the cards that make you win. There are four Kings and four Eights in the deck, so you have eight outs. Outs are cards that give you the winning hand, probably. Remember that you do not know for sure what cards your opponents hold.
Is eight outs good? There are 52 cards in the deck, and you have seen five of them: your two hole cards A-J, and the flop Q-10-9. 52 cards take away 5 leaves 47 cards unseen. 8 cards out of those 47 give you a probable winner; therefore your odds of winning are 8/47 or roughly 1 in 6. At this point, you have to take into consideration the number of opponents who are still in the hand and the size of the pot.