My father had/has it. It is simply a narrowing if the "tube" that runs down the spine where the spinal cord is. When it narrows, it eventually rubs on some nerves in the cord yielding pain the the area where that nerve attaches. So although a pinch occurs say in the middle of your back, the pain may feel like it in in a lower part if your legs.
My father had surgery for this where they go in and mechanically open up the canal where the cord sits. You want a well-rested physisian as hes holding the spinal cord in 1 hand and a scalpel in the other carbing like in a pumpkin. Once done and healed up, good to go!! But his grew shut again 12 years later and he had to repeat the process. That was 20 years ago as he's 89 now. And it's starting to do the same thing again. No surgery again for him at that age...he just sits down after standing for maybe 10 min when it starts to hurt. Sit down for 5, then good to go for 10. Still plays golf as many times a week as he can get a kitchen pass from my mother!!