What EXACTLY is the Mark of the Beast? Explained by the BIBLE!

What does the BIBLE say the Mark of the Beast is? Alot of people believe the mark could be a chip the vaccine, or having something to do with AI but what if these are distractions? I myself used to believe in these theories until I was informed on what exactly the Bible says.

2 Peter 1:20-21
Knowing this first, that no prophecy of the scripture is of any private interpretation.
For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.