What is the meaning and reasoning behind the Open/Closed Principle?

Question

The Open/Closed Principle states that software entities (classes, modules, etc.) should be open for extension, but closed for modification. What does this mean, and why is i

Answer 1

The principle means that it should easy to add new functionality without having to change existing, stable, and tested functionality, saving both time and money.

Often, polymorhism, for instance using interfaces, is a good tool for achieving this.