What is mild bilateral facet disease? in my lower back.

It is : Facet joints are the joint structures that connect the vertebrae to one another. It's not uncommon through normal "wear and tear" - so called degenerative arthritis - that these joints break down a bit. It's a very common finding and doesn't mean that you will have any problems from this now or in the future. Though it could be causes some discomfort in you; everyone's body is different.

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 radiofrequency ablation would be helpful for you.