Pot odds
Pot odds are the ratio between the current size of the pot and the amount you'd have to call, used to decide if a call is worthwhile.
Pot odds compare the price of a call to the reward you stand to win. If there are 100 chips in the pot and a rival bets 50, you'd be calling 50 to win a 150-chip pot, which is 3-to-1. You can also read it as a percentage: you're risking 50 to win 200 total, so the call needs to win about 25% of the time to break even.
They come up most often when you're holding a drawing hand and someone bets into you. Before calling, you check how cheap the call is versus how big the pot is. The bigger the pot relative to the bet, the better the price, and the more often a speculative call makes sense.
For a quick beginner habit, turn your pot odds into a break-even percentage, then compare it to your equity, which is your rough chance of ending up with the best hand. If your chance of hitting is higher than that break-even number, calling tends to be a good deal over the long run; if it's lower, folding is usually the cleaner play. These are practice-chip decisions about reading a single spot, not betting systems.
Example
You hold a flush draw on the turn with two of your suit on the board; the pot is 200 chips and your opponent bets 50, giving you 5-to-1 pot odds (calling 50 to win 250). With one card to come you have about a 1-in-5 chance (roughly 4-to-1 against) of completing the flush, so the 5-to-1 price beats your 4-to-1 odds and the call is a clearly good deal.