These are rapid back-and-forth shuttles with one step in between direction changes. Go two steps to the right, then two to the left.
This really focuses on lateral change of direction, and it's a great way to improve coordination and leg strength/power while building structural integrity in the lower body. Higher reps will scorch your quads and lungs.
Ever had one of those days where you just couldn't get into the flow of your workout? Or worse yet, you don't even make it to the gym? It's happened to all of us before. The mind is willing, but the body just isn't ready (or vice versa).