In Texas Hold’em and Omaha poker variations, a backdoor straight is a straight a player completes using both the turn and river cards. Another similar term for this type of draw is “runner-runner.”
For example, a player has a 7, 9 hand. The flop produces an A, K, 8. The flop didn’t produce anything of value, but the player continues in hopes of making a good poker hand.
The turn produces a 5, then the river comes up a 6, helping the player line up a straight and the best possible hand in this situation.