As you can see the circles represent the health of each host as determined by the applicable monitors running on that host. High Memory and High CPU are two system monitors that are included by default for every host. The remaining monitors are from services that are running on the host.
You can zoom in and out using the onscreen controls  or with the mouse scroll wheel. An important feature of the hostmap view is the ability to re-organize all the hosts by the Tag :
All the tags available for that hostmap can be seen in the drop down menu. Make your selection to regroup the hosts accordingly:
Clicking on a host or host group will refocus the Hostmap on that selection. It will also populate the contextual menu on the right and display events limited to just what you've chosen:
Hovering the mouse over any single host on the hostmap for more than one second will bring up an info bubble:
Now, you may want to investigate how to add an integration as a data source to further increase the utility for your specific application.
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