Just Starting... in the "dumb questions" category

I know N-O-T-H-I-N-G… what does the “Run the Tests” button do? When I select it, I get nothing.

Below the “Run the Tests” button, you will see a list of test cases. These exercise various requirements of the challenge.

When you run the tests, each of these will be marked depending on whether the tests pass or fail. Below all of them are passing. In the output box to the right, you can also see more information about the failing tests:

Here you can see I’ve run tests again after changing the code. Some tests pass, but not all: