I just finished a load of wash in my washer. There is debris along the top of the washer
Barrel. What could that indicate? The clothes weren't full of debris when I put them in.

Check where the water supply comes in to make sure the debris is not getting pumped in with the water when the washer fills? That's my guess/suggestion, however, I'm no expert in such issues.

Can you better describe the debris. You may have thought all the pockets were empty but my kids have proved me wrong more than once. It is not likely coming in with the fresh water, the valve and/or screen would plug up first.