Reachability statistics provides insigh on the quality of your hotspot's Internet connection.
You can easily see if (and for how long) your hotspot was unreachable and the response time to the ping requests.
This data is collected when you enable "Instant Alerts" in the app.
A good connection has a low deviation and low response duration. HeliumGeek checks your hotspot from a US location, hence hotspots out of US will be having a bit higher response time in these charts. In any case, you should aim durations less than 200 milliseconds. For most of the hotspots in US, this value is less than 100 milliseconds.
Deviation is the difference between the max and the average duration values. A stable connection should have low deviation (something less than 100 milliseconds).
Using Wi-Fi for your hotspot or problematic Internet connection may result high deviation values.