I think they meant if you got metal fouling in the barrel (like shavings of jacket, bits or filings off the cases or whatever, if you dont get most of these out, you might fire a round which cant push this stuff out of the muzzle in front of the projectile, so the projectile will "overtake" the crud and probably bulge the barrel in the process. It might be so small a bulge that your eye cant see it, wherever it is along the length of the barrel, you will lose overall muzzle velocity, and if it is in the last quater of the barrel (muzzle end) your accuracy will suffer.
From what I have heard, water is as bad if not worse. if you have water in your barrel and shoot, the projectile can try to "overtake" the water, which has no where to go and cant compress, so you can end up with a nice bulge in your barrel too
Muzza