The informed minority in a game is a faction whose members are aware of each other. Contrast the "uninformed majority", which is much larger but its members have to find out who else is in their faction.
The informed minority usually has a means of whittling down the majority (or Town), such as a factional kill that one of its members can use at Night.
It is possible for multiple informed minorities to exist in a single game - two Mafia groups, for instance. However, it is frowned upon for a single player to constitute the informed minority - for instance, some would argue that a game with only a Town and a Serial Killer is not Mafia.