[The beast] forced all the people, small and great, rich and poor, free and slave, to be given a stamped image on their right hands or their foreheads, so that no one could buy or sell except one who had the stamped image of the beast’s name or the number that stood for its name.
Wisdom is needed here; one who understands can calculate the number of the beast, for it is a number that stands for a person. His number is six hundred and sixty-six (Rev. 13:16-18).
The Bible uses a lot of symbols and images in its expressions. The book of Revelation particularly used several images and theological themes. It used most of these to convey events that will culminate in the ultimate triumph of Christ. I think the beast symbolizes the many emperors who killed and persecuted the early Christians but mainly Caesar Nero, whose name in Hebrew has the numerical value of 666. Some believe that his “mark” was the coins of the Empire that had his image.
The truth is there is no official explanation from the Church as to the real meaning of this revelation. This is because when it comes to revelations, most times we cannot know for certain what it means until the events begin to unfold. However, this is the closest and most reasonable explanation based on the background study of the conditions of the author and his times.