A stunt, also called a twist, is a move on the defensive line that involves two players. The two defensive linemen, usually a defensive tackle and a defensive end, exchange roles in their gap responsibilities. The design is meant to confuse offensive linemen and their pass protection or assignments in the run game. The two most common types of stunts are: End/Tackle (E/T) stunts and Tackle/End (T/E) stunts.
On E/T stunts, the end crashes inside to the B-gap in an effort to free up the tackle, who loops around outside. In more simple terms, the end goes first, and the tackle goes second. The end has to be strong enough to occupy both the offensive tackle and the guard. His job is to occupty the offensive tackle inside and prevent them from being in position to pick up the defensive tackle wrapping around. This is where you can sometimes see defensive holding on the line. The defensive end knows, he needs to prevent the offensive tackle from moving outside, so they may try to grab jersey and prevent them from reacting to the stunt.
T/E stunts are the opposite. Now, the defensive tackle is the one crashing outside and the defensive end is looping inside. The defensive tackle wants to occupy the offensive guard and pull him outside. When executed well, that leaves space for the end to loop inside and pressure the quarterback.