What is the Open-Closed Principle, and what does 'open for extension, closed for modification' actually mean in practice?

A class should be extendable to support new behavior without changing its existing, already-tested source code. In practice, this usually means depending on an abstraction (an interface) and adding new behavior by writing a NEW class that implements it, rather than adding another if/else branch to an existing class — e.g. adding a new PaymentMethod by creating a new class implementing a PaymentStrategy interface, instead of editing a giant switch statement inside PaymentProcessor.