The hip flexors are a large muscle group on the anterior (front) portion of the hip joint. Quite frequently, due to the increased amount of sitting in our modern culture, this muscle group lacks length and flexibility. Contrary to much popular belief, it is a tightness of the hip flexors that contributes greatly to low back pain (not hamstring tightness). But stretching the hip flexors isn't as easy as it sounds.
The muscles that make up the hip flexor are the iliacus, psoas major, and rectus femoris. The iliacus has attachment points on the pelvic bowl and the femur, allowing it to flex the hip joint. The psoas has attachments at the lumbar spine and the femur, allowing it to both flex the hip joint and extend the lumbar spine. The rectus femoris is one of the quadricep muscles, it both flexes the hip joint and extends the knee.
With regards to what most consider tight hip flexors and the cause of low back pain, the psoas is the culprit because it is the only one that directly attaches to the lumbar spine. When we sit all day, we develop shortening of the hip flexors, as well as of the distal (lower end) of the hamstrings. It's common to hear people people blame tight hamstrings for their low back pain, but in truth the hamstrings are actually lengthened and weakened at their proximal (upper) attachment point at the hip (again, due to sitting). The sensation of tight hamstrings often comes from a weak hamstring's attempt to fight against the overly shortened and overly developed quadriceps and hip flexors. Lengthening the hip flexors, therefore, will not only help alleviate back pain, but can also alleviate the feeling of hamstring tightness (by helping correct posture and length/tension relationship in the hip musculature).
So how do we stretch the hip flexors? First, we need to be able to place the hip into extension. For many people, finding hip neutral will be a challenge and may be enough of a stretch to start with. The overlooked part of stretching the hip flexor is what is happening above and below the hip. Because the psoas attaches to the lumbar spine, the lumbar spine must be stabilized in order to stretch it. And because the rectus femoris attaches to the tibia, the knee joint must also be stabilized. This means that a neutral lumbar spine should be locked into place (via contracting the transverse abdominis) to maximize the lengthening of the psoas and the knee must be flexed to maximize the lengthening of the rectus.
One of the most popular poses in yoga to stretch the hip flexor is a low lunge with the back knee on the earth. This is a fantastic posture, but it is often done with a large arch (lordosis) in the lumbar spine. In order to properly stabilize the proximal end of the psoas the lumbar spine needs to be in neutral. Change this one simple cue about this posture and the entire functionality will change as well.