The Test is done at many different frequency so to produce the graph of the audio path response.
The purpose of the Test is to compare the audio path gain of a system composed of an amplifier and loudspeaker but you can test also only a loudspeaker or earphone. The phone generate a list of tones(frequencies), the signal is acquired from the microphone and the relative power is calculated for each frequency. Please note that 0dB is an undetermined value so the measures are relative and not absolute values.

A teacher used this App to do an experiment in his class, he was able to find the speed of sound using a phone and a cardboard tube. Determining the loudest frequencies, then these are the resonance frequencies and will lead to the speed of sound because the length of the tube is related to the wavelength of the resonant frequencies.