Single Responsibility
In plain terms
A class or module should have only one reason to change.
Going deeper
The Single Responsibility Principle says each unit should do one thing, so changes to one concern don't ripple into others. One job per unit.
Under the hood
The 'S' in SOLID; improves cohesion, testability, and maintainability.
Examples
SOLID, separation of concerns, focused classes, maintainability, cohesion.
Think of it like...
Like a specialist who does one job well rather than a jack-of-all-trades.