Definition: A condition whereby excessive friction between high spots results in localized welding with subsequent splitting and a further roughening of rubbing surfaces of one or both of two mating parts.
Pretty common with high chrome stainless steels. Early autolading pistols with SS slides and rails were infamous for it. It is preventable by coating the contact surfaces with a soft metal base anti-sieze compound like moly lube. Oil or grease work too but tend to work off from the surface sooner. You can easily identify a metal surface that has been galled, it is very rough like Tim said perhaps even with a few sharp burrs and there will be a lot of friction with what ever part it mates to.