What does the Dependency Inversion Principle (DIP) mean?

The Dependency Inversion Principle has two rules: (1) High-level modules should not depend on low-level modules — both should depend on abstractions. (2) Abstractions should not depend on details — details should depend on abstractions. Without DIP, a ReportService (high-level) might directly instantiate a MySQLDatabase (low-level), creating tight coupling. With DIP, ReportService depends on a DatabaseInterface abstraction, and MySQLDatabase implements that interface. You can swap in PostgreSQLDatabase or MockDatabase without changing ReportService. DIP is the foundation of dependency injection and is critical for testability.