What is the value hidden behind the letter O in the SOLID principles?

The letter "O" in SOLID principles stands for the Open/Closed Principle. It states that software entities (classes, modules, functions) should be open for extension but closed for modification.

This means that the behavior of the system can be extended without changing the existing code, which reduces the risk of errors and makes maintenance easier.

Example: if you need to add new functionality, it is better to create a new class implementing the required interface than to modify an existing class.