Active Monitoring is the way to check the site performance by simulation of visiting by real users. It is done by external service, usually with distribyted networks. The checking servers create the requests like the real clients, who are trying to download a web page. Doing it on regular schedule, allows to check if the page is available from the web. If not - there is a possibility to catch and report the error. The difference between the bot and a real person is visible only for bulky pages with lots of content - in order to save the traffic, monitoring systems usually download only a part of such pages, which is enough to make sure the site is Ok.
Active monitoring is an independent eye on your site. The checking is not connected to your company and web hosting, and so provides the clear third-party view of the situation. HostTracker service uses active monitoring to supervise web sites.