The Unity debugger monitors which lines of code are being executed as the tests are run. It uses this data to compile a report on how much of each method of each class has been executed for the assemblies involved in the testing. By adding the Code Coverage package and enabling it, debugging data will be collected and reported upon when you run Unity tests.
As shown in Figure 13.33, the Unity Code Coverage tool generates a set of web pages to inform us about how much and which lines of our code are being examined with our unit tests. While even 100% coverage does not guarantee the code is "correct," a high percentage of code coverage does indicate that the behavior of most of our code is being tested to some extent.