Is a vivid and crude way of describing severe trauma or injury to someone's testicles (potatoes), implying they have been so badly crushed or damaged that they are no longer in their normal shape or condition. This expression is used to emphasize the extent of the injury and the extreme pain associated with it.
Guy 1: Fuck he crushed his nuts on the rail!
Guy 2: I bet he has balls like mashed potatoes now!