This is simple example for mocking usage.
There are some sources on the 'src' folder -- these would be the real implementations that provide the final image going to your microcontroller.
On the tests folder on the other hand are the unittest sources -- these are the tests that you wrote to verify that the modules work as you wanted. And that they work together as you wanted.
Unfortunately the example is quite small, and you dont really see the true gain of unittesting, you need bigger and more complex project for that, and those projects are then not good examples of the test framework.