Google previously announced that it would be banning unregistered devices from running Google Apps like the Play Store and other services, called GApps by the modding community. This was a move to prevent companies from installing the Play Store on phones that Google hasn’t approved.
Unfortunately, modified devices are also “unregistered.” Google allowed you to register your devices with custom software, but it was a pain and there was a 100 device limit (which might have hurt a few developers, though most people would never hit this limit).
Now Google has removed this device limit and has made the registration process easier. All you’ll need is your Google Services Framework ID, which you can get from apps like Device ID. Then your phone is good to go!