Your hamstrings are groups of muscles and tendons between your hips and knees. If you pull a hamstring while running or jumping, you'll feel pain at the back of your thigh. Ouch!
The hamstring muscle group is one of the most powerful in your body — attached to the knee and hip, these are the muscles and tendons that make it possible for you to run, walk, and jump. It's fairly common for athletes to injure their hamstrings and be (temporarily) immobilized. Fittingly, hamstring is also a verb meaning "render powerless." So an astronaut might complain that a lack of funding will hamstring NASA, making planned Mars voyages impossible.