I'll try to give you a simple example.
The flop and turn have already been dealt. You have the Q-10 of hearts.
The board is now K-J-8-4 and the 2 low cards are hearts.
The pot has $100. You are heads up with 1 person left. The other person acts first, and he goes "all-in" for his last $50.
Should you call? You are quite sure your opponent has at least a pair of Kings.
Now we can figure out the "Pot Odds".
First - how many "outs" do you have to win? You will assume that all 4 Aces and all 4 Nines will make you win the pot. Plus there are 7 other hearts with which you can win. (I already counted the 9 and Ace of hearts, so don't count those again.)
So that is 15 ways you can win. There are 46 'unknown' cards left in the deck. 31 of these cards you lose with, and 15 you win with. Your odds of winning, then, are 31 to 15. This is approximately 2 to 1. If the pot is offering you 2 to 1 or better, then you can call.
How much was the pot offering? There was already $100, plus the $50 that was just bet. That's $150 total. You can gain $150 pot by putting $50 more. That is THREE to ONE. So your odds to win are 2 to 1, but the pot will pay you 3 to 1 if you win.
Should you call? Yes. How do we know? Because the "pot odds" show you it's worth it.
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Important - understanding "Pot Odds" is much more important in "Limit" poker. When you're playing "No Limit", "pot odds" only help you when someone is "all-in" like my example above. Much more important is understanding "Implied Odds", but I'm going to let you learn about that from the first poker book you read. In a nutshell, not only do you need to know how much is in the pot compared to what the bet was, but you need to know how much your opponent has left in his stack, AND how much more of that stack are you likely to win if your winning card hits.