What it means when the HTTP sensor is up

The UP status of an HTTP sensor means that the webserver delivers an HTTP result that is correct according to the HTTP protocol and that the URL is available. This means that the webserver software is up and running, but you do not know whether the results are correct, e.g. the webpage can contain error messages. So you don't know whether the CGI scripts, etc. work correctly or whether, e.g. the database of the webserver is ok. It is recommended to also check the content of a webpage by using the HTTP Advanced Sensor instead of the simple HTTP sensor for added reliability.