It’s pain that starts in your one side buttocks and run down your leg it’s very painful, I have it off and on the one thing that helps me is flat ice pack and lay down with it on my butt and stretching that part of your pain plus an inflammatory pills

The main nerve that runs from back and down leg gets pinched by the discs in the back or the muscles in the groin. In the groin, it's the piriformis muscles. Google that and there's lots of info on it.