What is SOLID? What is it used for?

SOLID is a set of five principles of object-oriented programming aimed at creating flexible, maintainable, and extendable code:

• S (Single Responsibility Principle) — a class should have only one reason to change.
• O (Open/Closed Principle) — software entities should be open for extension but closed for modification.
• L (Liskov Substitution Principle) — objects of subclasses should be replaceable with objects of the base class without affecting the correctness of the program.
• I (Interface Segregation Principle) — prefer many specific interfaces over a single general interface.
• D (Dependency Inversion Principle) — high-level modules should not depend on low-level modules; both should depend on abstractions.

Applying SOLID helps write code that is easier to test, maintain, and extend.