Private APIs are APIs that only Apple is allowed to use. They let you do stuff you're not supposed to be able to do like put stuff on the home screen, however,

Private APIs are APIs that only Apple is allowed to use. They let you do stuff you're not supposed to be able to do like put stuff on the home screen, however, these days, it's mostly enforced by the security model, so you can call the private API all you want and it's not going to do anything. They also let you do things like record what other apps, etc are doing. Think of the Linux top command, on iOS there's no real reason for an app to need to list the other processes, so you can't. However, if you want to know what apps are running on the phone this would be a very useful API to have access to.