When unit testing code there is a really powerful feature that is either called mocking or faking or sometimes using stubs and what these mean is creating objects that look like real objects but aren’t.

Why would you want something that looks like something but isn’t? Unit testing means breaking your code into small chunks and testing that the small chunk does what it is meant to. Testing small chunks of code in isolation is difficult as they often call other chunks of code so we test and pretend that the other thing is correct and to validate those other small chunks, we unit test them.