Definition of runner-runner
A "runner-runner" in poker is when we are helped on the turn and the river by two cards. For example, if we have A5 in spades on 2-8-K with a single spade on the flop, if the turn yields a card in spades and the river also yields a card in spades, we would say we made a "runner-runner" flush. If you have a flush draw on the flop (let's say there are two spades on the flop instead of one), you will hit your flush about 36% of the time. But if there is only one spade, you will hit your flush about 5% of the time, which is a marginal draw.
It is better to play your runner-runner in position
Running a backdoor (runner-runner) is marginal. But having a runner-runner can add to the equity of our bluff if we are in position. Imagine the following scenario. We are in button position. All players have folded before us. We open and the player in SB position calls. We have A5 in spades and the flop falls 2-8-K with a single spade. This is a nice flop to bet in position. We have folding equity, plus a runner-runner. We bet and our opponent calls. The turn is a 4 of spades! Since the card opens the backdoor, it gives us plenty of equity to bluff the turn again!