High-level modules should not depend on low-level modules. Both should depend on abstractions.

Without DIP (tightly coupled):

class OrderService {
  private db = new MySQLDatabase(); // Direct dependency

  save(order: Order) {
    this.db.insert(order);
  }
}

With DIP (loosely coupled):

interface Database {
  insert(data: unknown): void;
}

class OrderService {
  constructor(private db: Database) {} // Injected

  save(order: Order) {
    this.db.insert(order);
  }
}

Benefits: Easier testing, swappable implementations, reduced coupling.