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That is your percentage of catching one of your outs. Example: You have two hearts. Two more hearts fall on the flop, so you have 9 outs. 9 x 4 = 36, giving you 36% chance to hit your flush and your opponent a 64% chance to hold up and win with a pair. 64/36 is a little less than 2 to 1. Therefore, it would make sense to call bets slightly higher than half the pot size. If another heart doesn't hit on the turn you are now 9 x 2 = 18% 18%/82% is a little worse than 5 to 1, meaning the bet has to be less than 20% of the pot.
After the flop determine the number of outs you have. Multiply that number by 4. After the turn you multiply your outs by 2.