Your hip flexors are a group of muscles reponsible for keeping your hips and low back stable, strong and flexible. Tight or weak hip flexors are more likely to lead to injury and degeneration in these areas.
This video demonstates a couple of stretches you can do to keep these muscles flexible. It is important to keep a neutral spine throughout these stretches and to do within your tolerance levels. 30 second holds are recommended. There should be a healthy balance between your hip flexors and hamstrings so I would recommend stretching both. The video for hamstring stretches were uploaded last week.
Good luck and happy stretching.