Function coverage is a metric used in software testing to determine whether all functions in a program have been executed during testing. It helps ensure that each function has been tested at least once, which can reveal potential issues or bugs. By measuring function coverage, developers can identify untested parts of the code, leading to more thorough testing and improved software quality. This metric is part of a broader category of code coverage metrics, which also includes line coverage and branch coverage. Tools that analyze function coverage can provide insights into the effectiveness of test cases, guiding developers in refining their testing strategies for better reliability and performance.