The Psoas muscle group (shown in the picture below) runs from the legs, goes through the abdomen, and then attaches to the spine.
When this muscle group becomes tight it pulls on the low back causing tension, pressure on the intervertebral discs, and results in hyper-lordosis and pain. As the pelvis becomes tilted forward and it may also be referred to as excess anterior pelvic tilt or lower cross syndrome if accompanied by tight paraspinal muscles combined with weak abdominal and gluteal muscles.
A foam roller can be used to release the front leg muscles (hip flexors).
After releasing the leg muscles with the roller you can roll out the psoas. I suggest the use of a small ball placed under the abdomen when lying face down. A tennis ball works fine. You have to feel for dull achy areas where the tension is highest then let gravity do the work.
After releasing the muscles then you can stretch them. This hip flexor stretch will also stretch the psoas muscle group.
Even better if you can arch back slightly while stretching and elevate your lower leg. Hold the stretches for 30 seconds to a minute each side.
Here's an alternate stretch for those with less coordination or pain.
Do the release techniques and stretches daily for the best results. If pain continues or worsens then make sure you have a proper evaluation by a licensed professional.
Here's a useful video: