This is the lying hip extension exercise, also known as the lying hip bridge. The muscles you will work during this exercise are the butt and hip flexors.
So, to perform this exercise correctly, lie on your back, with your feet flat on the ground and your knees bent. Next, brace your abs and contract your glutes, as if you were squeezing something between your cheeks (this pre-contraction is very important for beginners). Now, bridge your hips up by contracting your glutes and then hold your hips elevated for a one count while bracing your abs and squeezing your glute muscles. Then, slowly lower your hips back down until they are an inch above the ground. Repeat.
Be sure not to use your lower back for this exercise. Although it will be tense, using your back is incorrect.
This is an excellent beginner exercise because it teaches muscle control. However, intermediate users should also use this in a bodyweight fat loss circuit, while advanced exercise users should use it as a warm-up. For instance, you can use this exercise at the beginning of a conditioning or warm-up bodyweight circuit.