The mark of the Beast is associated with the number of the Beast, 666, and the name of the Beast in Revelation 13:17.
The following is an excerpt from the United Church of God booklet The Book of Revelation Unveiled:
"The second beast of Revelation 13 will cause 'all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads, and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name' (Revelation 13:16-17).
"What's behind this strange-sounding practice? How will it affect people as the time of the end draws near?
"There is historical evidence that, in the ancient Babylonian culture, slaves 'were identified…[by] some kind of mark, either tattooed or branded on the face or the back of the hand' (H.W.H. Saggs, The Greatness That Was Babylon, 1962, p. 173). There may be parallels between this ancient practice and what God reveals will happen in the end time.
"Many have speculated as to the meaning of this puzzling prophecy. Some interpret it in terms of supercomputers, unique personal identity numbers and even microchips imbedded under the skin.
"However, we should consider that receiving a mark on the hand or forehead could well be figurative. Remember from Revelation 7:3 that God's servants were sealed on their foreheads. And in Deuteronomy 6 Moses told God's people to 'keep all His statutes and His commandments,' noting, 'You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes' (Deuteronomy 6:2; Deuteronomy 6:8). The hand and forehead here represent one's actions and thoughts respectively—which should be dedicated to obeying God.
"Likewise the mark of the Beast is evidently a mark of disobedience to God. Many Bible students have pointed to God's seventh-day Sabbath as an identifying sign of His people in Exodus 31:12-17 and see the mark of the Beast as running counter to it. They believe that forced observance of Sunday—a day originally dedicated to the worship of the sun as a god instead of the observance of the seventh-day Sabbath—could be involved. This view is strengthened by the fact that other prophecies in the book of Revelation show a great counterfeit religious figure and system will play major roles in leading people away from the true God and play into Satan's hands at this time.
"The Bible does not explicitly define the specific methods that the Beast will use to identify those free to participate in commerce at the time of the end. Some sort of advanced surveillance may be involved…
"However this prophecy is fulfilled, it is clear that it will involve compromising and contradicting God's commands. Having proof that one is officially associated in some way with the Beast will be necessary for one to be allowed to engage in the commerce."