A Replacement is a player who was not originally in the game at the start, and instead, as the name suggests, replaced a player who for whatever reason could not continue playing. Almost every game of Mafia needs at least one Replacement. Being a Replacement is difficult since it often requires "catching up;" likewise if you've been a spectator, you might need to scratch many of your suspicions once you replace in and find that you are scum.
Note that occasionally a Replacement can throw the balance of a game if not done with consideration for player skills. For instance, if an inexperienced townie is replaced with a very skilled player, especially late in the game, the Mafia may cry foul as this dark horse unravels their web of deception. But in such a scenario the Mafia have no legitimate way to prevent this Replacement barring killing an inexperienced townie earlier in the game (which is radically opposed to most mafia strategies); thus simply a Replacement can cause a game to become grossly unbalanced.