A more thorough explanation:

A continuation-in-part application is a type of patent application that is filed by the same applicant during the pendency of an earlier application. It repeats a substantial part of the earlier application but adds to or subtracts from the claims. This type of application contains new technical descriptions from the inventor or reflects improvements made since the parent application was filed.

For example, if an inventor filed a patent application for a new type of car engine and later discovered an improvement to the engine, they could file a continuation-in-part application to claim the improvement as well.

A claim in a continuation-in-part application is entitled to the benefit of the parent application's filing date if the claimed subject matter is the same, but new matter takes the filing date of the continuation-in-part application. Continuation-in-part applications are usually filed to describe and claim later-discovered improvements to an invention, or to distinguish the invention from some prior-art reference.

Overall, a continuation-in-part application allows an inventor to add new information to an existing patent application without having to file a completely new application.