Something is not quite right, but the system has not yet reached a critical state. Something serious is wrong with the host. By selecting one of the shown hosts you will be taken to a page that holds detailed information about the services and devices being monitored on the host. Services shows the current status of the monitored services, e.g. web or mail server. Devices holds information about the host's devices, e.g. free disk space, CPU load.
Incidents is a list of previous reported messages such as warnings and alerts.
Host Info describes the host's operating system, CPU type and similar things.