Function Coverage is a software testing metric that measures whether each function in a program has been executed at least once during testing. It helps ensure that all parts of the code are tested, which can identify potential bugs or issues. By focusing on function coverage, developers can gain confidence that their code behaves as expected in various scenarios. This metric is often used in conjunction with other coverage types, such as statement coverage and branch coverage, to provide a more comprehensive view of testing effectiveness. High function coverage indicates thorough testing, while low coverage may suggest areas that need more attention to improve software quality.