This is an easy way to prank your friends and pretend you have an app capable of detecting body odors. the principle is based on a scientific fact that each smell has a distinct chemical compound ( a set of atoms).
So what this app does is the following:
The smell goes through the speaker opening behind your phone, the app then scans and analyzes the smell compounents and show you the chemical charachteristics on the screen.
But the best part is : no smell is shown unless you tilt the screen a little forward , this is the trick so you better keep it a secret ;)