Roy Tang

It means what the reminder text says. You can choose to cast Murderous Cut for a reduced cost (as little as one black mana!) by exiling cards from your graveyard. Each card you exile reduces the cost by (1)

Some decks will use the graveyard as a resource, so they will have cards that allow you to move cards directly from your own library to graveyard (often called “milling”). The idea is to win before running out of cards in the library, taking advantages of the additional bonuses provided by having more cards in the graveyard (for Murderous Cut, the bonus is having the choice to play it at a lower cost)