They are functions (in the programming sense) that can be called by other code, but Apple reserves them for their own use. They don't necessarily have anything to do with privacy, though some of them do. The "private" refers to them not being publicly documented and not provided for developer use. Internal developers at Apple are allowed to use them, but only as appropriate. Consider a function (API) that allows one to wipe the phone. Apple doesn't want just anyone to be able to write an app that provides that feature. Same with a lot of other functionality. Other private APIs are things that Apple might someday offer to developers, but hasn't yet. For example, Apple can change the icon of its own apps (note the calendar app, which shows the current numeric month day, and the clock app, which shows the time including moving second hand) but it does not offer this functionality to developers. [Edit: I was wrong about this; see firloop's comment below.] If they did, the results could be off the rails, visually, and make the platform look bad, so I expect when they do allow this, they'll do it with a heavy review process in place... but that's just one small example. A lot of private APIs are more under the hood housekeeping stuff that you just don't want the average programmer to mess with.

The API to change an app's icon is not private (as of iOS 10.3)

https://developer.apple.com/documentation/uikit/uiapplicatio...

However, it's much more restricted than the functionality natch referred to. An app can only change its icon while running, and even then only with the user's explicit permission.

Oooh, nice! Thanks.

> Apologies for my laziness, but what is meant by "private API" here and why Apple does not want them to be used?

Private APIs are not meant for use by third-party developers. They are not publicly documented and are subject to change without warning.

Often the private APIs are used to track users, because they allow getting identifying serial numbers from the phone.

Apple markets privacy as one of the features for the iPhone.