Compatibility Testing Checklist is a tool used to ensure that applications, software, and hardware are compatible with each other. It helps in reducing the risk of incompatibility and ensuring that the final product is of the highest quality. The checklist contains tests that are conducted and analyzed to measure the compatibility between the components, such as browser compatibility tests, operating system compatibility tests, and device compatibility tests. It also involves security testing to make sure that the application is secure. The checklist helps in identifying the existing issues and fixing them before the project is officially released.