Purpose: This is the best exercise for low back injuries. It is the precursor to nearly every exercise you will do on a regular basis. Some people refer to it as core stabilizing, bracing, or engaging their core. The bridge is a pelvic tilt followed by a contraction of the glutes and hamstrings to lift the hips off the ground until a straight line is achieved from your shoulders to your knees.
Instructions: Lay flat on your back with knees bent, posteriorly tilt (try to roll your hips backwards). Bring your ribs down by tightening your abdominals then breathe out. If done correctly you will feel your abdominals tighten and your low back press down against the floor.
Just note that although it seems like a simple exercise a lot of people have difficulty with it and the only way to improve is to continuously practice it.
Common Errors: There is an arch in the back when you are tilting - indicative of weak (and improving!) lower abdominals, or tight restricting hip flexors. In the bridge, the knees buckle together - indicative of weak hip stabilizers. To correct this issue, try doing them with a light band around the knees and gently push outwards against the band.