The monitor page shows some basic information about the alerts:

Status Guide

The following checks are performed every five minutes:
Application status.
The internal state of your Mendix App is monitored in several ways. See the monitoring documentation page for more information.
CPU check.
This will cause a warning if your app exceeds 90% CPU usage and critical when it is at 100%.
Application/Database Server Up.
Ping for network connectivity on the app node / database node.
Webserver.
Whether your application is reachable over HTTP(s), the same way normal clients would connect to the app via their browser.

Disk usage alerts are set at:  warning=80, critical=90