I don't believe there is any neuroscientist on the planet who would claim to know exactly what each brainwave is for! (yet!)
As far as I'm aware, current research seems to lean heavily towards Alpha as being the best indicator of "calm", so with Interaxon's team of neuroscientists and many years of research, that would be my best guess at what is getting calculated under the hood of their Calm app.
If you use Muse Monitor to record yourself in a "normal" state and then try relaxing, you can see that your average Alpha level will be higher during the relaxation period. This is most clearly shown when taking averages for many sample sets.
In the "Virtual Dream" research paper linked above, they have a graph showing this trend, averaged across their sample set: