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.
Now it's time to take a closer look at the smallest functional unit of training parameters: the repetition. If you build your repetition quality, you'll reap more gains from your workouts. That's definitely a good thing!