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.
The term comes from Magic: the Gathering, so named after the card Millstone.