RPC-1 term is used to refer to PC's DVD-ROM drives that were built before 1st of January, 2000.
Before that date, virtually all DVD-ROM drives permitted the user to change the region code settings of the drive freely when necessary, thus allowing watching/using DVD-Video movies from all over the world.
This was changed when the MPAA forced hardware manufacturers to implement a new technology, dubbed as RPC-2 in their drives, that only allows max. 5 changes in the region code.