The Rule of 2 and 4

The rule of 2 and 4 is a useful way of estimating your odds a winning a particular hand.  Put simply, for an all-in decision on the flop, multiple your "outs" by 4.  Otherwise (i.e., on the flop or turn), multiple your "outs" by 2.

Your odds are the number of unseen cards that causes your hand to be a winning hand.  For example, if you have AJ, the flop doesn't pair you, and you think your opponent has a pair of tens, you have 6 outs (3 aces and 3 jacks).  That means, you have an approximately 24% chance of winning an all-in on the flop, and a 12% chance of your hand improving on the next card.

The rule of 2 and 4 is approximate and not totally accurate, but it's surprisingly useful.

To learn more, Google is your friend.  Also consider this book: Essential Poker Math, Expanded Edition: Fundamental No-Limit Hold'em Mathematics You Need to Know