DLC is short for "DownLoadable Content". Basically add-ons and expansions to the main game, as Fil explained. Usually you have to pay extra for it but sometimes they let you download new stuff for free. Thanks again for all the Outlast stuff, Fil!
dlc's are a sort of game expansions such new outfits new weapons and so on, but in this case is a prequel of the main game.. what happens before the normal game.. also they added new enemies and characters..