Runner-runner is a slang term gaining credence as a common descriptor. It describes a player catching two running cards in a row in order to make their hand. This is generally only used in games with a flop, turn, and river, to describe a player needing to catch a favorable turn and river in order to make their hand.
Runner-runner is an adjective applied to a hand, as in "I'd need to make a runner-runner straight", or is less commonly used as a noun to describe the theoretical two cards that would be needed, as in "I'd need runner-runner to make my straight".
Consciously playing to hit runner-runner is, naturally, not a good strategic move. The odds of catching two cards in a row to make your hand are usually quite small. Runner-runner flushes have odds of no more than 4% of hitting; runner-runner straights have odds of less than 1%. Bad players and calling stations often stay in hoping for runner-runner hands but lose much more money in the long run than they win when they make their hand.
On the other hand, even good players sometimes back into a runner-runner hand, such as when they have
and the flop comes
Right now, a good player believes they probably have the best hand, with top pair and top kicker. However, it's possible that someone has a better hand - for example, another player with a pocket pair of nines would now have flopped a set. The player with top pair will probably not fold (in a limit game, anyway), and if the turn and river bring two more hearts that don't pair the board:
then the player with the AK has just beaten the player with trips by catching the runner-runner flush, even though the AK player wasn't necessarily sure they needed to hit it to win. While the player with trips may feel that the other player played badly, in fact they didn't; they just accidentally caught runner-runner.