The Mark of the Beast in Biblical scripture is a mark put on the hand or the forehead without which one cannot buy or sell. The passage about the mark of the beast as well as the number 666 comes directly from the book of Revelation.
15 The second beast was given power to give breath to the image of the first beast, so that the image could speak and cause all who refused to worship the image to be killed. 16 It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, 17 so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.
18 This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666.