If there is $80 in the pot, and the bet to you is $20, then you're getting 4 to 1 for your $20 bet: 20x4=80.

If you think you have a better than 4 to 1 shot of winning the hand, then it's a good bet, vis a vis an overlay.

People seem to get confused by the difference between pot odds and pot equity. The bottom line is that pot odds are a "fact." Pot equity is an opinion.

In the aforementioned eg. your pot odds "are" 4 to 1. You "think" you chances of winning the hand (pot equity) is better than 4 to 1.