In normal operation, a rolling-element bearing has the rollers and races separated by a thin layer of lubricant such as grease or oil.  Although these lubricants normally appear liquid (not solid), under high pressure they act as solids and keep the bearing and race from touching.
If the lubricant is removed, the bearings and races can touch directly.  While bearings and races appear smooth to the eye, they are microscopically rough.  Thus, high points of each surface can touch, but "valleys" do not.  The bearing load is thus spread over much less area increasing the contact stress, causing pieces of each surface to break off or to become pressure-welded then break off when the bearing rolls on.
The broken-off pieces are also called wear debris.  Wear debris is bad because it is relatively large compared to the surrounding surface finish and thus creates more regions of high contact stress.  Worse, the steel in ordinary bearings can oxidize (rust), producing a more abrasive compound which accelerates wear.