Test coverage is a measure of the amount of source code executed by the test cases. It is an important metric to determine whether the unit tests are sufficiently thorough for the code. Test coverage closely relates to the cyclomatic complexity of the program. A program with higher cyclomatic complexity requires more extensive test cases to cover all possible execution paths adequately.
A higher test coverage indicates more thorough testing and thus correlates with reduced occurrence of bugs and faults in production. Most codebases employ automation to ensure that coverage does not go down with any change.