It's a thickening of the base of the neck in bottle-necked cartridges, which can result in poor accuracy, or even excessively high pressure, because of the "doughnut" being pressed around the bullet when the cartridge is chambered.
It normally occurs for two reasons:
The process of firing and resizing the cartridge. This tends to move brass forward in the case, and the top of the shoulder (which is thicker than the neck) starts to "migrate" into the bottom of the neck.
Necking cartridges up in caliber, say making .260 Remington cases from .243 Winchesters. Again, the top of the shoulder becomes the base of the neck.
But I have also occasionally encountered new factory brass that had slight doughnuts.
