Mod-confirmed information is anything that is known to be absolutely true due to information that comes directly from the moderator; it's the highest level of confirmation possible in a Mafia game.
For example, an Innocent Child's alignment is mod-confirmed to everyone, and an Informed player's information is mod-confirmed to that player. Dead players normally have mod-confirmed roles and alignments due to the flip.
Results from investigative roles can be treated as mod-confirmed in a Normal or Open game in cases where it's known that no role capable of interfering with the investigation could exist. However, due to things like Millers, action results are normally given a status a little lower than mod-confirmation, and are treated as simply confirming a player.