Didn't understand a thing with interfaces

So my learning journey was going really well for the most part, that was until I reached the Interface lessons. I do know what an interface is, kind off, but the explanation in the lesson “Interfaces and Testability” just knocked me down. I just didn’t catch up, there was so much explanation and a new thing called Unit Testing with new syntax and stuff.

I am a complete newbie in C#, I started learning it about 2 weeks ago, so if there is someone who can give me a more simple, for dummies explanation of what just happened in this lesson, I will really appreciate it.