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.
You've spent the spring and summer training to look good on the beach. As a result, you now have the chest and arms of a silverback gorilla, but the thighs and calves of a pink flamingo. Time to fix that.