Peak indicators

These controls are the three led indicators placed just above the dock, at the left, at the center and at the right of the screen. They're contextual to the currently visible modules. The first one is related to the audio input of the left module, the second one monitors the output of the left module, which is also the input of the right module, and the third one refers to the output of the right module. Every indicator will turn on with a red light when a high peak is detected in the relative audio signal, i.e. the sound was too loud.
If a led turns on, you could notice a degradation of the quality of the audio when it passes through an effect module (audio source modules don't distort the sound). When this happens you should adjust the parameters of the modules (usually by turning down the volumes), so to avoid the peaks going through the effects. In general it's better to turn up the main volume of your device rather than the volume within the app, which should remain at a proper level throughout the chain of modules. In practice, however, you may ignore some occasional blinks of the peak indicators: what counts most is your ear.

Notes

At the end of the chain there is always a hidden compressor. Just like a regular Compressor module, it will remove all the incoming peaks (because it's also a so-called "limiter") but it may introduce some noticeable distortions too.