SOLID Principles in Delphi [3] — The Liskov Substitution principle

The Liskov Substitution Principle in Delphi! I start with the official definition:

Subtype Requirement: Let Φ(x) be a property provable about objects x of type T. Then Φ(y) should be true for objects y of type S where S is a subtype of T.

Still here and wondering what that means? Good, I had some trouble understanding this…