Beginner: Plank is a fantastic pose for beginners to strengthen their abdominal muscles and spine, as well as arms and wrists. Make sure to distribute your weight evenly across your hands and energize the abdominal muscles. If it is too much at first, you can bring your knees to the floor.
Advanced: Advanced yogis can enhance the benefits of plank pose by continuing to widen across the collarbones and pulling the shoulders away from the ears. Check your alignment: is your spine really straight? Is the core engaged?