You need to phone your doctor and ask for more explanation, but at a guess, and only a very uninformed guess, it may be that you have had small kidney stones and these have broken up naturally leaving some bits in your bladder. The question you need answered is why the kidney stones occurred and what problems this is posing for the health of your urinry tract.
some years ago I had terrible trouble with gallstones, which passed by themselves (v unpleasant!).
when I had my gallbladder scanned they said there were a lot of "silt like" products in there, and that they were either stones breaking down or stones building up. fortunately I had no other problems.
not sure if this applies in your case but it sounds similar. I hope it's something as simple.
Lil O'lady is quite correct, just ask the Dr. what he means by "debris" I cannot imagine you passing debris or small stones in your urine, without knowing about it...e.g pain or "gritty" feeling in your urine. Always a problem...abnormal results of investigation in a symptomless patient. Don't worry, you are 75 so you havn't done too badly.