The "Mark of the Beast" is a visible, and possibly digital symbol of people's allegiance to the "Beast", aka the AntiChrist, who will rule in the last days. This mark will be required for anyone who wants to buy or sell, or make use of any modern convenience. With this mark the AntiChrist will be able to control all trade, commerce, money, resources, and people. If you don't take it, you risk death at the hands of the AntiChrist and his minions. But if you do take the mark, you'll be relegated to torment in Hell forever. Refusing the mark and aligning yourself with Christ through Salvation ensures that you only suffer briefly now but live in comfort in Heaven forever. Taking the mark and having temporary comfort ensures that you will suffer forever in Hell. Trust me, don't take the Mark. Better yet, get saved now BEFORE the rapture so you don't have to face such a decision.
Another thing to note is that, while the mark isn't officially in use today, the final prototype stages and technologies are being put in place right now to facilitate its rise and use later on. Some of these include digital payment systems, electronic tattoos, inserted RFID microchips, and even mind control. Yes, mind control. Part of receiving the mark, as best can be determined by theologians, involves not just receiving either a physical tattoo, an electronic identifier (ie, RFID chip, electronic tattoo, etc) or even possibly both, but also the impartation of mind control. It's uncertain how this will be done, only that it will happen. This will be done to ensure total, unswerving loyalty to the AntiChrist. There are numerous technologies right now that are being developed that might fill that need. But none of them are at "release stage" yet, meaning that at least a few more years are required to perfect them.
If you're reading this and the rapture has happened (see the "rapture" entry here on the site for more details on that), whatever you chose to do, I strongly advise that you DO NOT under ANY circumstances take the mark of the beast. You will forever be cast into Hell if you do, and that is NOT where you want to spend eternity. Choose Jesus Christ because, even if you suffer for a time down here, it will be brief and you will eventually be given great peace, comfort and rewards in Heaven forever. If you're reading this and the rapture hasn't happened yet, and you're not saved, I strongly recommend you do. Seek out Jesus Christ while He can be found. If you'd like to talk about Jesus or getting saved, please feel free to contact me. :)
(*Do you have a better description, or did you find an error? Please let me know.