Auto percussion thing triggered by short loud sounds. For example, you may clap your hands, stomp with feet, or click fingers to trigger it. Could be connected to speakers (app has algorithm that disable self triggering).
Or you can play acoustic guitar and Stomp Trigger will accentuate the loudest strokes.

Also it's a BPM counter. You can beat some rythm to findout it's beats per minute speed.
Note! Stomp Trigger adapts to background noise but very loud background noises, that overload your device's microphone for a long time may deafen this app for a minute.