Your hamstrings and where they are anatomically, governs how you pelvis is aligned. If they are short they will tip your pelvis backwards this increases the pressure on the low back. It tightens the gluteal muscles or renders them weak and ineffective (more of this later!
If you are a runner then these become tighter and prone to injury or transmits too much force down the leg giving you calf muscle problems or Achilles/foot problems.
If you are an office worker then this keeps them shortened!
It is a difficult muscle to stretch but so worth it. Here are some I recommend that won't irritate your back and will stop you from accidentally cheating!
This stretch should be done with the remaining knee bent to prevent injury to the lower back. Initially you will not be able to extend the stretch as far as the image shows, you should only ever stretch to a point that is comfortable for you.
This stretch can be a bit hard on the knees so we would suggest using a yoga mat.