The open-closed principle declares that you should be able to extend a class' behavior, without modifying it. This principle is raised by Bertrand Meyer in 1988:
Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.
A program depends on all the entities it uses, that means changing the already-being-used part of those entities may just crash the entire program. So the idea of the open-closed principle is straightforward: we'd better have entities that never change in any way other than extending itself.
That means once a test is written and passing, ideally, it should never be changed for newly added features (and it needs to keep passing, of course). Again, ideally.
