The side to side drill helps improve your running form by working the abductors and adductors, and improves lateral strength and coordination. Important things to keep in mind when performing this drill:
Stay on the ball or forefoot.
Push off from the inside of the trailing foot.
Pull with the leading foot.
Keep your toes pointing forward. Your leading foot will have a tendency to point outward, in the direction of motion, try and bring it back in so it points forward in the direction you are looking.
View this video to learn how to perform this drill correctly: