New matter rejections occur in patent prosecution. When a patent application is filed, it can be thought of as a photograph of the invention taken at a moment in time. No new matter can be added to the patent application after that moment. However, if the drawings filed with the patent application depict a part of the invention that was not discussed in the text of the application, the text can be amended to include a description of the depicted matter and such additional text is not considered to be new matter.
If an invention is improved after a patent application is filed, the improvement is new matter and cannot be inserted into the existing patent application. The remedy is a Continuation-In-Part application.