The Liskov substitution principle is the L in the well known SOLID acronym. The original principle definition is:

Methods that use references to base classes must be able to use objects of derived classes without knowing it.

At first glance this principle is pretty easy to understand. At second glance it seems redundant with the OOP concept of polymorphism. After all, the whole point of polymorphism is to consume an abstraction without knowing the implementation behind isn’t it…