This muscle is also called the hip flexor. The illiopsoas muscle attaches onto the bones in the low back. When it is tight, it will contribute to forward tipping of the pelvis and increase the curve in the low back. This increased curve strains the joints of the low back leading to pain.
While standing, put one foot in front of the other in a track stance. The front leg should have some bend and the back leg should be straight. Then, lean forward by bending the front leg while keeping good posture. You should not be bending at the waist while doing this stretch. You should feel this stretch in the front part of the thigh on the leg that is in the back.
There should be no pain. If there is pain, lean into the stretch less. Then switch to the other leg.
Hold the stretch 20-30 seconds. Repeat the stretch on each leg 2-3 times daily.