Hip flexors are the muscles which attach to the front of the hip, and when tight, cause forward tilt/rotation of the pelvis which can lead to joint and muscle issues elsewhere in the body.
The hip flexors are commonly tightened due to sitting for long periods of time, putting the muscles in a sustained shortened position.
Staying active or making sure you're not immobile for long periods is the best preventer, failing that, these stretches should help lengthen and loosen the muscles.