For an application to be considered 'well'-tested, distinct functionality should ideally have its own separate unit tests where it's tested against the different conditions you expect it to work under. All tests must pass before functionality is considered 'complete'. This allows developers to both modify a unit of code and it's dependencies with a level of confidence about whether these changes have caused any breakage.
As a basic example of unit testing is where a developer may wish to assert whether passing specific values through to a
sum function results in the correct output being returned. For an example more relevant to this book,