Activation Lock is a feature designed to protect your Mac in the event of loss or theft. It prevents a new user from getting full use of the Mac because they will still appear in your list of devices in Find My utility, ready for both locating and remote erasing.
Apple itself describes the function on the Support page so:
Activation Lock prevents others from using your Mac if it's lost or stolen.
Activation Lock protects your Mac if it's lost or stolen, and increases your chances of finding it. With Activation Lock, your Apple ID password or device passcode is required to use Find My? deactivate, erase your Mac, or activate and use it again. Even if you erase your Mac remotely, Activation Lock prevents others from activating it again without your permission. All you have to do is "Where is?" Keep it enabled and remember your Apple ID and password.
Basically, a Mac protected in this way can only be used to a limited extent and thieves in particular are unlikely to have much fun with the device.
If you now want to sell your Mac used or give it away, it is advisable to deactivate this activation lock before passing it on. This works like this:
Alternatively, you can also log out of the iCloud completely via the Apple ID → iCloud system settings, which also disables the Mac activation lock.
Important: If you are planning to give your Mac away, there are a few more things you should do to ensure that no information about you remains on the Mac and that it is completely separate from your Apple ID. You can find instructions on how to do this for the new Macs in the article Reset M1 Mac for sale and erase data. It explains the individual steps in an easy-to-understand manner.
Jens has been running the blog since 2012. He appears as Sir Apfelot for his readers and helps them with problems of a technical nature. In his free time he drives electric unicycles, takes photos (preferably with his iPhone, of course), climbs around in the Hessian mountains or hikes with the family. His articles deal with Apple products, news from the world of drones or solutions for current bugs.