Hip flexors include iliacus, psoas major and minor, rectus femoris and sartorius.
These muscles can become tight from sitting for long periods of time or overuse (running, cycling, heavy lifting, etc.)
I recommend ALL my clients (all levels of active including non) to stretch hip flexors at least 3 times per week.
Tight hip flexors, amongst other things, can make you feel like you have low back pain.
Kneel a foot on the floor, the other knee on the floor.
If you don't have a mat or carpet, use a folded towel under the knee for comfort.
Push your hips forward, keep chest up and open, focusing on the target muscles for stretch.
I grabbed my foot for an added stretch.
S T R E T C H