A hamstrings stretch is a stretch of tight hamstrings muscles at the back of the thigh.
You can feel the hamstrings tendons on either side at the back of the knee. Those on the medial (inner) aspect are the semi-tendinisus and semi-membranosus. On the lateral (outer) aspect is the biceps femoris.
The hamstrings muscles contract to flex (bend) the knee and extend (straighten) the hip. Tight hamstrings often make a sportsperson prone to injury, or a knee patient prone to re-injury and hamstrings stretches are an important part of knee rehabilitation.