The HiFi Data Explorer is a window into the HiFi API. This is a web frontend to the API which makes it really easy to quickly access the advanced features that the API allows.
There are two tabs at the top. The Query tab allows you to type a HiFi API query and display the results below. The script tab allows you to directly manipulate the data using the JS API. This can be really handy when, for example, chaning the email notifications of every form on a site.
This is also a helpful tool when working with HiFi Theming. It lets you live-test queries that you might use in the templates. If you have your Data Explorer open now, I would encourage trying a few queries to see how easy it is.