HotZones are areas of very high aircraft traffic as determined by historical ADS-B returns.
These areas can be imported into any EFB (eg. Garmin Pilot/ForeFlight/FltPlan Go on iPad or Android) that supports KML/KMZ user overlays.
What is it showing?
HotZones are highly processed air traffic data - its important for you to know what it depicts
Focused around TACs
The data is focused around Terminal Area Charts (class B) airspace since it is generally busier in those areas and also to give a local view of what is considered busy. New York and Salt Lake City are not the same "busy"
No traffic in controlled airspace
You are under ATC observation and therefore much safer so traffic from inside of A.B,C,D airspaces are filtered out
Busy, where you are
Hot Zones shows only greater than the second standard deviation from the mean for traffic density in that specific area below 5000' AGL of the median airport elevation (where most uncontrolled conflicts occur), this means only relevant very high traffic density areas will show to minimize the clutter and increase the usefulness of this information