The pelvic tilt is a common core exercise that also provides a gentle stretch to the lumbar spine. This particular exercise is called a posterior pelvic tilt (vs an anterior pelvic tilt) because the top of the pelvis translates posteriorly with this movement.
Begin on your back on a mat. Bend your knees so that both feet are flat on the floor. Note the slight arch in your low back.
Tuck your pelvis so that the arch of your low back flattens into the floor and your belly button moves upward slightly towards your rib cage. Next, gently engage your abdominal muscles as you tilt the pelvis. Hold for 1-2 seconds. Relax to the starting position.