Have had leg nerve pain, got mri, showed mild arthritis in facet joints. what are they?

See below: The facet joints are cartilage covered joints that link one vertebrae with another. They guide the motion of the spine. With age they do get more arthritic but do not always cause pain. Pain from facet joints is usually achy and can certainly radiate down into the leg. Usually the pain from facet joints goes into the buttocks, back of the thigh and sometimes to the upper calf. Worse with standing.

Small joints : Every level of your lower back hasca set of 2 facet joints that help to bear load, allow for motion and restrict motion as well. Like any weight bearing joint, they can degenerate or wear out leading to arthritis of those joints which can be a cause of back pain or even be a factor in leg pain.