Milling is an informal game term. Milling means to discard cards from the top of a deck. "Mill" on its own implies discarding cards from your opponent's decks, but some cards "self-mill," discarding cards from your own deck for some benefit. Dwarven culture specializes in both, with cards like Dwarven Axe (1C9) and Realm of Dwarrowdelf (2R12). Mill strategies often combine with discard strategies, because forcing your opponent to discard cards means they draw more cards when they reconcile, burning through their deck more quickly.
Self-mill decks often use cards like Úlairë Nertëa, Messenger of Dol Guldur (1U234) or Preparations (7R12) to fetch lost cards from your Discard Pile. Self-mill decks also often use cards like Ring of Artifice (17R4) or Úlairë Lemenya, Dark Enemy (19P37), which are empowered by spotting cards in your discard pile.