Mill is a keyword action that moves cards from the top of a player's library into their graveyard. When a card instructs you to Mill a card, you put the top card of your library into your graveyard. If it says Mill three cards, you put the top three cards of your library into your graveyard. The player who mills is determined by the card's wording; you might mill yourself or an opponent.
Milling matters because it fills graveyards, which can fuel abilities that care about cards in the graveyard, such as flashback or delve. It can also be a win condition: if an opponent's library is empty and they try to draw, they lose. But milling an opponent can also give them resources if they use their graveyard, so timing and targets matter.
