A gutshot straight is a straight filled from the inside. In other words, the missing card is in the middle of the straight, as the player already has four of the five needed cards, usually two on each end.
For example, a player has a 6, 7 in their hand. The community board reads 3, 4, Q, K after the fourth street, and then the river produces a 5. In this case, the player has completed a gutshot straight on the river.
This hand is also known as an inside straight, and you can read more about it in our extensive article on the topic.