A gutshot, or inside straight draw, is a hand one card short of a straight, where the missing card is in the middle of the sequence. Only four cards of one rank can complete it. That makes it a weaker draw than an open ended straight draw, which can be completed by eight cards. The chance of hitting a gutshot on one card is small, so it is usually worth paying only a small price to see the next card.
Example
A player holds 5 and 6 and the flop shows 7, 9 and a king. Any of four 8s makes the straight. With 47 cards unseen, the chance on the next card is 4 in 47, about 8.5%.