SOLID is an acronym that represents a set of five design principles aimed at making software designs more understandable, flexible, and maintainable. These principles are particularly useful in object-oriented design and programming. When followed properly, they can help in reducing tight coupling, making the system more modular, and easing the refactoring process. The SOLID principles are Single Responsibility, Open/closed, Liskov substitution, Interface segregation and Dependency inversion:

Single Responsibility Principle (SRP)

A class should have only one reason to change. This implies that a class should only have one responsibility or one job. By adhering to this principle, you minimize the chance that changes in one part of the software will affect unrelated parts. This in turn results in a lower risk of introducing bugs and makes the system more maintainable.

When classes (or even functions and modules) take on multiple responsibilities, it often becomes harder to modify, understand, and maintain them. These multifunctional classes often lead to what is known as "code smells" – indicators that the code might need refactoring. One common code smell is a class that's grown too large because it's trying to do too much.

By focusing on a single responsibility, you can ensure that:

• The class has a clear, well-defined role.
• Changes to the class will only be required for one reason, reducing the potential impact of changes.
• The class can be easily understood, tested, and maintained.