You have a flush draw after the flop — two of your suit in your poker hand, two on the board. You're last to act. Everyone checks to you. Bet. Sometimes you'll win the poker pot immediately without a struggle, but even if you don't, you'll frequently have helped your cause.
Now, everyone is apt to check to you on the 4th board card (the turn). If you make your flush, you just keep betting, natural as natural can be. If you miss, you check along. And the great thing is that you get a free card which could have costs double in common limit games where the size of bets increase after the flop. The final (river) card is also effectively free, because if you miss, you'll usually fold.
There's another twist to this tactic. You don't want to overuse it, because astute players may catch on and adapt, but one of the built-in tools of deception comes from mostly betting these flush draws when you have at least one card higher than the board. That way, you have additional chances of making top pair and continuing to bet on the turn.
When this happens, many opponents won't notice at the showdown that you were originally betting the flush draw. They sometimes just see the top pair and forget when you made it or how. This psychologically camouflages the fact that you're often betting flush draws “on the come,” hoping to get a free card.