Three muscles on the back
Your hamstrings are three muscles with long names: the biceps femoris (FEM-or-is) on the outside, and the semitendinosus and semimembranosus on the inside.
Most of them start at the bones you sit on, at the bottom of your hips. They run down the back of your thigh and end just below your knee.
Try it: sit on a hard chair. Feel the two bony spots under you? That's where your hamstrings begin.
A funny name
In very old English, 'ham' meant the back of the knee. The 'strings' are the strong tendons there.
Try it: sit down and bend your knee. Feel behind your knee on both sides. Can you find the tight cords? Those are hamstring tendons.
People also love to tell a story that butchers long ago hung up hams by these same strong tendons.
Bend and push
Your hamstrings and your quadriceps are partners. The quadriceps straighten your knee, and the hamstrings bend it.
Your hamstrings also pull your leg back at the hip. That pushes you forward when you walk and run.
Try it: hold onto a chair and bend one knee to bring your heel up toward your bottom. Feel the back of your thigh tighten?
Why stretching matters
Hamstrings can feel tight, especially after a lot of sitting. Gentle stretching after you play, when your muscles are warm, can help them feel loose and comfy.
Try it: sit on the floor with your legs out straight. Slowly reach toward your toes until you feel a gentle pull. Hold it and breathe. Never bounce, and stop if it hurts.
It doesn't matter how far you reach. Bodies bend in different ways, and that's okay.