A hamstrings graft is a ligament transplant using one of the hamstrings tendons from the back of the knee.
Illustration from the back of the knee showing the tendon of the semitendinosus - which is accessible from an incision on the medial side. The tendon of the gracilis is not shown but runs alongside.
Usually the ACL surgeon will create a 4-stranded graft using the semi-tendinosus - often together with another muscle, the gracilis (which is itself not actually a hamstring).
Illustration from the back of the knee showing the