Specific yoga poses targeting the hamstrings can alleviate tightness and improve flexibility. The hamstrings are the three muscles that run along the back side of your thigh, connecting your pelvis to your knee. Many people have tight hamstrings as a result of repetitive movements or poor posture.
There are a lot of ways to end up with hamstring tightness. Sometimes it just comes down to anatomy. Regular stretching, even beginning in childhood with activities like dance and gymnastics, helps, but most people don't do enough to maintain their flexibility.