SOLID
In plain terms
Five object-oriented design principles for maintainable code.
Going deeper
SOLID stands for Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion, guidelines that reduce coupling and make code easier to extend and change safely.
Under the hood
Guidelines, not laws; applied dogmatically they cause over-engineering, but they meaningfully reduce coupling and ease change in evolving codebases.
Examples
SRP: one reason to change; OCP: extend without modifying; DIP: depend on abstractions.
Think of it like...
Like five building codes for software: follow them and the structure stays sturdy and easy to renovate.