This is another doctrine that is being mishandled in these last days.
It comes from Revelation chapter 13.
To understand what the mark of the beast is, you must first understand what the beast is. To understand the book of Revelation you must realize that it is full of symbolical and figurative language.
If you want to truly understand every detail concerning this doctrine of the beast and the mark it forces on the world, you should read through every commentary on Biblehub for chapter 13 of the book of Revelation, going from one verse to the next.
I will teach you in layman's terms, because that's the best I can do, what the beast and the mark of the beast is.
The beast is the persecuting world power. The beast has seven heads, each head being a different world power from a different age in this world. The sixth head was the pagan Roman Empire, this head was wounded with a mortal wound.
"The wounding the head being the abolishing pagan idolatry; and the healing of the wound, introducing popish idolatry." -Matthew Henry commentary on Revelation 13:3
The seventh head being the revived Roman empire under the rule of the papacy from Rome.
"Another beast arose, being like that of the first beast." Being very similar having the same spirit of the world but in a new more subtle disguise. -Quote from unknown author (please contact me if you know so I can credit)
"The second wild beast is that change which is a change of mode, but not of spirit-a change of manners, but not of heart; there is more refinement, more civilisation, more mind, but it is still the world-power which is worshipped; it is the self-seeking adoration of pleasures, honours, occupations, influences which spring from earth and end in earth-the pursuit of powers which are worldly." -Ellicott commentary on Revelation 13:11
This beast is the world we live in now (the seventh head of the beast with a change of mode (second beast)). It is a world wide dominion. This antichrist/worldly sensual spirit prevails over every kingdom on earth. All countries (kingdoms) are under the power of Satan in these end times (proof).
So now you know that this beast's heads are the persecuting worldly powers that have been from age to age, rebelling against God, against truth and against the saints of God. And that we are now living under the dominion of the last of these heads which is the revived Roman empire under the rule of the papacy which has undergone refinement. But no doubt the second beast has risen. Just look around you.
The mark of the beast, is not a literal physical mark. Just as the seal of God (Revelation 7:3) is not a physical mark on the people of God.
"That seal we understood as spiritual, in the faith and in the character; this evil brand we must interpret in like manner." -Ellicott commentary on Rev 13:16 Being also a spiritual mark.
You cannot receive the seal of God without your willingness to serve the Lord and keep His commandments. Nor can you receive this mark of the beast without willingness to do its commandments and taking an active role in fighting against God and His saints. A mark that must be spiritually discerned. (read the Holy Bible and keep God's commandments and precepts and you will gain this discernment by knowing right from wrong)
There is much fear surrounding this doctrine where there needs not be. Nobody can unknowingly receive this mark. Those who receive this mark, are made partakers of the benefits of Satan's/the beast's spiritual dominion. But by becoming willing servants unto Satan and his spiritual dominion, have sadly, guaranteed their place in the lake of fire.