A hamstrings graft is a ligament transplant using one of the hamstrings tendons from the back of the knee.
Tendons involved in a hamstrings ACL graft
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).
ACL Reconstruction Basics: Quadruple (4-Strand) Hamstring Autograft Harvest. Frank RM, Hamamoto JT, Bernardoni E, Cvetanovich G, Bach BR Jr., Verma NN and Bush-Joseph CA. Arthrosc Tech. 2017 Aug; 6(4): e1309–e1313.