the application will compare your Avg. to the published results from the site https://www.fueleconomy.gov and show a score that tells how close your Avg. to the published one.
the results are not 100% accurate, it depends on combined Avg (City, High way) but it's a basic indicator for your Avg.
if you get a very low score, may be you should consider consulting a maintenance shop.
This app for now is for Hybrid Vehicles only.
more brands could be added later.