MILD DEGENERATIVE FACET JOINT CHANGES BILATERALLY AT L5-S1

COULD SOMEONE EXPLAIN WHAT DOES IT MEAN, PLEASE?

SYMPTOMS: TAIL BONE IS PAINFUL WHEN SEATING TOO LONG (I.E. TAKING A LONG FLIGHT), PRESSURE AROUND THE TAIL BONE WHILE SEATING (UNCOMFORTABLE FEELING) BUT NO PAIN, AND WHEN LAYING ON BED ON MY BACK (THE LOWER BACK NEAR THE TAIL BONE AREA, AGAIN, IS A LITTLE PAINFUL, BEARABLE, THOUGH).

THANK YOU FOR TAKING SOMETIME TO ANSWER MY QUESTION.

It means that the wing-like projections of bone that stick out on either side of the back bones at the place where your spine and pelvis meet are slightly worn down. These normally have cartilage on the surfaces that rub the adjacent vertebra during any change in the position of your spine. When these surfaces are healthy, they allow smooth painfree movement.
For many people with this condition, the cartilage wears away over a long period of time leaving roughened surfaces.
These roughed edges become inflammed causing lower back pain. Typically, it's worse first thing in the morning or if you try to get up after a prolonged period of inactivity. It usually improves when you get moving but worsens again with prolonged activity and also in the evening.
If you have tailbone pain it may be related to the process of degenerative changes in your spine or may indicate something else going on. I note that you posted this a year ago. If you are not under the supervision of a specialist, it might be useful to see your GP again to review the symptoms and possibly repeat the X-Rays to see if there are any further evidence of changes since facet joint changes are chronic and often progressive. Any tingling, numbness or even loss of control of bowels or bladder should prompt you to see the GP asap.