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