Template variables help make your monitor alerts more meaningful. You can use any host tag you have set in the agent as a template variable. After (1) defining the metric and (2) setting up your alert conditions, use your Host Tags in the (3) Say what's happening section.
In the example below I am using the `owner` tag, which was defined in the agent using `owner:scott`. There's a handy reference for template variables accessible by clicking the Use message template variables link.
When triggered, the host tags will now display in your alert.