As a developer, what this means to me is:

For every piece of code you write, there should be a unit test covering it that you can run fairly instantly with 1 keypress which catches at least internal-consistency errors. (Integration tests would cover external-consistency errors, but when you're refactoring/adding/deleting code, internal-consistency is violated far more often than external consistency, in my experience.)

Once you experience this for yourself, you will NOT want to go without it. It allows you to IMMEDIATELY get back to coding without waiting for test runs to finish and without creating bugs unknowingly that you only catch much later.