The second beast of Revelation 13—the two-horned beast that arises from the earth—"causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead, so that no one can buy or sell unless he has the mark, that is, the name of the [seven-headed] beast or the number of its name. This calls for wisdom: let him who has understanding reckon the number of the beast, for it is a human number, its number is six hundred and sixty-six" (verses 16–18).

Notice that the "mark" is the same as the "name" of the beast, and the "number" is the "number of its name," which is 666. The mark/number, therefore, is the name of the beast. To have the mark (name) of the beast on "the right hand or the forehead" is to wear the beast's mark of ownership. This means simply that the person who voluntarily receives the mark has formally acknowledged his loyalty to the Antichrist system/leader described as a seven-headed, ten-horned beast that initially arose from the sea.

Whether the "mark" turns out to be a tattoo or an implanted chip or something else is unimportant, in my view. What is important is that the person receiving it will be aware that he is submitting his will to an ungodly system, so he will be involved in voluntarily persecuting the saints.

You will note that the next chapter speaks of those who have God the Father's name written in their foreheads (14:1). This shows their allegiance to the Father, just as the mark (name) on the foreheads of those who follow the beast shows their loyalty to the beast. The most important thing here is the contrast between the two groups—the followers of the beast and Satan and the followers of the Lamb and His Father. And, no doubt, one of the purposes of this vision (chapter 14) is to encourage the latter group, which is persecuted by the former, by giving them a glimpse of the final destiny of the faithful.

In ancient times, the first beast was the Roman Empire, while the second beast was the imperial cult that promoted the worship of the emperor. At that time, the symbol ("mark") of loyalty was a reef on the front of the home or a little bit of incense burned to demonstrate one's allegiance to the emperor god. Failure to abide by this could result in economic deprivation or worse. We can see how this would be problematic for followers of Christ. We don't know what manifestation this "mark" will take in the end time, but we do know that it will involve a conscious decision to obey man rather than God.