The theory of pot odds is simply one that compares the return you will get on your bet, vesus the odds of you winning the pot.

Let's say you are holding K-Q (that is to say, suited). Worth a call, right? The flop comes J-T-3. So you have an open-ended straight draw. You need an Ace or a Nine to catch a strait - and you are pretty sure a straight will win the hand.

The pot already contains 500 chips. Someone bets 150 chips. Should you call it, on a straight draw? Here's the pot odds calculation...

Chances of catching a straight on the turn or the river: 34% or 2.90 to 1
Pot odds: 150 to win 650 chips or 4.33 to 1

So you have a 2.9:1 chance of winning and will receive 4.33 times your bet. So it is definitely worth it.

But what if someone bets 300 chips? Now the pot odds are 2.66 to 1. Since you still have a 2.9:1 chance of winning, but will only receive 2.66:1 on your bet, pot odds says that it is not worth it. So fold.