Thomas,

I'm sure more knowledgable guys (Micky, Duggboy etc) will put this better than I can... but in the mean time... Its when the action of a break action gun (double rifle or shotgun) gets "loose" and has play even when closed between the barrels and the action. All doubles will wear, but with "cheaply made" guns, and high pressure rounds (like most of the rimless cartridges: 375 H&H, 416, 460 Wetherby etc) this will happen much, much sooner.

Hope this helps!