A hammer is a vote on someone which results in the majority needed to eliminate them. Once the hammer vote is placed, the game moves into Twilight until the mod posts the death scene. The player who has been hammered is considered beyond salvation as of the moment the hammering vote has been placed; barring the influence of a role like a Governor nothing will stop them from getting eliminated. Many mods allow hammered players to post in Twilight but some do not.
Some players may use the threat of a hammer vote as a way to gambit in hopes of pulling votes off of them with a last second with "Oh well, go town." or "I wasn't lying, I really was the doctor."