The facet joints glide and allow the spine to move freely with support. Facet arthrosis is degeneration or wear and tear of the facet joints. Severe facet arthrosis would be advanced degeneration, possibly with arthritis or bone spurs. This might cause pain, and/or cause stiffening of the spine, and/or even put pressure on the spinal nerves or thecal sac (which contains the spinal cord). I've put more information in the links below.