Dredge is the Keyword Mechanic of the Golgari Guild in Ravnica: City of Guilds and usually worded as "Dredge [X]". If a player draws a card and a card with Dredge is in the graveyard, the player may choose to instead put the top X cards from his or her library into his or her graveyard and return the card with Dredge to his or her hand.
However, the most popular Dredge card is Life from the Loam which itself can refuel easily expendable land cards such as Cycling Lands or Fetchlands and put more of those lands into the graveyard using Dredge. With this the card can fuel other cards such as Solitary Confinement, Seismic Assault or Retrace cards such as Raven's Crime.
Outside Ravnica, Dredge has only been printed in Future Sight's Dakmor Salvage.