The Mark of the Beast is described in Revelations chapter 13.
And (the Antichrist) causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads:
And that no man might buy or sell, save (except) he that had the mark, or the name of the beast, or the number of his name.
Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.
Rev 13:16-18 KJV
According to this passage in the Bible, the Antichrist will force everyone to accept a mark in either the right hand or forehead of the wearer. Without this mark you won't be able to buy or sell. The Beast is identified by the number 666.
Nearly 2000 years ago the Apostle John, who prior to meeting Jesus had been a simple fisherman, predicts that a global economic system will one day be instituted in which everyone will be forced to receive a number, without which they will not be able to buy or sell. It is only since the advent of computers and electronic banking that this prophecy could be fulfilled.