What is the "short shuttle run"?

According to SB Nation, the "short shuttle run" is an exercise performed at the NFL Combine and is designed to test an athlete's explosiveness. It showcases an athlete's lateral quickness, ability to change direction and agility.

The test is performed by having a player sprint 5 yards to the right and touch the ground, sprint 10 yards to the left and touch the ground and then sprint 5 yards back to his original position. Any player that is able to perform this drill in 4 seconds or less is generally considered very impressive; several top performers often score around the 3.8-second range.