In Delphi unit definition interface section is used to declare any public sections of code that appear in a unit.
In OOP jargon, you can think of an interface as a class with no implementation.
If you are up to creating a rock solid application in a way that your code is maintainable, reusable, and flexible the OOP nature of Delphi will help you drive the first 70% of your route. Defining interfaces and implementing them will help with the remaining 30%.
Read the full article to learn how to What's an Interface? Defining an Interface. Implementing an Interface
Others would like to know: Have you ever created (and implemented) your own interface? Why?