What does the letter 'S' mean in SOLID principles and how is it applicable to software system design?

The letter "S" in the SOLID principles stands for Single Responsibility Principle. It states that a class should have only one reason to change, meaning it should be responsible for a single specific task.

Applying this principle helps create more understandable, maintainable, and testable code. If a class performs too many functions, it becomes more difficult to modify and debug, and changes in one part can affect others.

For example, if you have a class that both processes data and handles output to the screen, it is better to split these responsibilities into two classes: one for processing logic and another for display. This simplifies system maintenance and expansion.