One of the best things about Android is that it's an open source project, so developers are free to edit it to create their own custom ROMs. Unfortunately that has led to the illegal distribution of Google's own apps, so the company has begun blocking uncertified devices from running them.
Google makes money by licensing its apps (including Gmail, Google Maps, Google Play Music and the Google Play Store) to manufacturers, letting them pre-install them on devices on the condition that they meet a set of stringent compatibility requirements. However, the apps are often distributed illegally on third-party sites – an activity the company wants to stop.
When a user tries to log into one of Google's apps on an uncertified device, they will now be shown an error screen advising them to return it to the retailer if it was sold that way, or register it with Google if they're running a custom ROM.
Registration involves tying the device to your Google account by logging in and entering your Android ID – a string of characters generated each time you perform a factory reset. You can do this up to 100 times, which will be fine for most people, but might prove awkward for hardcore Android fans.