The bridge is a great exercise that targets mainly the glutes and hamstrings. But when done properly, you can also work on your abs and adductor muscles (inner tights).
So don't let your lower back overarch, and try to sqeeze your legs together for maximum benefit as shown in level 2.
Learn more about this pose here https://www.mindbodygreen.com/0-17705/5-reasons-to-do-bridges-every-day.html