A straight is a poker hand, where the 5 card ranks are all in a row - A to 5, 2 to 6, 3 to 7, 8 to Q, 9 to K, 10 to A, and so on.
An inside straight is a straight that is missing one of the middle cards - meaning that only four cards could be drawn to make the straight, meaning it’s a lot harder to make than an outside straight, which is missing the card from one of the ends, and therefor could be finished with 8. (A, 2, 3, 4 or J, Q, K, A are both considered inside straights as well, even though the missing card is from the end, since there’s nothing below the A in the first case or above it in the latter.)
Playing an inside straight is usually a bad plan, and it’s generally considered impressive when it pays off.