What is mild facet and ligamentous hypertrophy?

Facet Arthropathy. Could be facet arthropathy/arthritis. There are joints in the spine where the bones articulate and can develop bone spurs and cause the clicking with pain typically with extension. I would consider seeing a pain/spine specialist to see if facet injections or radio frequency ablation would be helpful for you.
Common aging changes. Hypertrophy (enlargement) of the facet joints and ligaments are common degenerative changes, seen with aging and wearing out of the spine. If they are mild, they likely are not causing symptoms, but this situation can be complicated. Your primary doctor or a good spine surgeon can help you figure out if these are causing problems for you.
Signs of aging. Thickening of the spinal support structures, a normal by-product of aging.