What is Runner-Runner in Poker?

Also known as backdoor, runner-runner means making a hand by hitting the needed cards on both the turn and river.

For example, if you hold 6♥ 5♥ and the board reads A♥ K♣ 7♠, you would need hearts to come on both the turn and river (running hearts) to hit a runner-runner flush. Getting a 7 and an 8 on the turn and river would give you a runner-runner straight.