The runner runner is a drawing hand where the player must hit a specific card on both turn and river to complete his combination.
For example, if you have AcQc against your opponent's set of fives on Ts 7h 5c, you have a runner runner straight draw and a runner runner flush draw to complete your combination. If the club comes on both the turn and the river, you will make a flush and win the pot, assuming it does not pair the cards on the flop. At the same time, if the turn is a king and the river is a jack or vice versus, you will also win the pot with a straight.
Runner runner draws are not really strong hands, and you should not be chasing them too much, but they can be great candidates to bluff yourself. You can keep barrelling on future streets if you get one of the necessary cards and put maximum pressure on your opponent. On top of that, another player will have a hard time putting you on your exact hand, so you might end up winning a huge pot if you hit it.