Pelvic tilts are an exercise comprised of very subtle spinal movements that strengthen the support muscles around the low back, particularly the abdominals
. They are a good preliminary exercise for those seeking low back pain
relief, and they feel great because they give your back a little massage. They can be done lying on the floor or standing with the back to a wall. The standing version is a bit more difficult, but it is a good option for pregnant women who are not comfortable on their backs or people who can't lie on the floor.
About.com Back and Neck Pain Guide Anne Asher has a great article on how pelvic tilts can help back pain.