This is your sciatic nerve!
It is the largest nerve in the body.
Sciatic pain can be caused from a variety of reasons. Perhaps an injury has damaged the nerve, or perhaps wear and tear means a bulging spinal disc is pressing on the nerve.
However sciatic pain may also be caused by muscle tension. The piriformis muscle, located in the buttocks, lies over the top of the nerve, so any knots or restrictions in the muscle fibres may put pressure on the nerve, triggering the pain.
The nerve runs all the way down your leg, so muscle tension anywhere along the path of the nerve could also contribute to sciatica.
Luckily muscle tension can be easily remedied with a massage!