A "hip bridge" is an exercise where you lie on your back and lift your hips up towards the sky while keeping your feet flat on the ground. It helps strengthen and tone your buttocks and lower back muscles. Full definition
Hip bridge with extension on Swiss ball This move works your core, hamstrings, and glutes. (health.com)
In the video for this workout, I talk a bit about form in hip bridge exercises and if you're new to working out, it's definitely worth watching. (pumpsandiron.com)