Bulging discs, pinched nerves, and strained muscles are just a few of the causes of low back pain. A brace helps by compressing the lower abdominal area, and the squeeze pushes up on the abdominal cavity helping offload some weight on the spine and providing relief. (think of toothpaste- when you squeeze the tube, the product pushes out of the top.
Same concept here.) A brace can also help by limiting painful motion while allowing the rest of the spine to flex, and also by supporting the front of the abdomen over a larger surface area of the back, giving the muscles a much needed break. Most braces are thin and lightweight, easy to fit and wear and hidden under clothing.