OCP. One of the principles of Object Oriented Design.
Bertrand Meyer first coined it in the first edition of Object Oriented Software Construction as follows:
A class should be open for extension, but closed for modification.
In other words, (in an ideal world...) you should never need to change existing code or classes: All new functionality can be added by adding new subclasses and overriding methods, or by reusing existing code.
This prevents you from introducing new bugs in existing code. If you never change it, you can't break it.