Is the app ready?
A load balancer shares incoming requests among several copies of an app. Health checks help it avoid sending requests to a copy that is not ready.
Liveness checks whether a copy is still working; if it fails, the system may restart that copy. Readiness checks whether it can handle requests now. A new copy may be running but not yet ready while it starts up.
A passing check has limits
A passing health check means only that the part it tested worked. An online shop might pass while its checkout is broken. Teams also use monitoring and smoke tests to catch problems health checks miss.