OnAudioFilterRead is called everytime a chunk of audio is routed through the filter (this happens frequently, every ~20ms depending on the samplerate and platform). The audio data is an array of floats ranging from [-1.0f;1.0f] and contains audio from the previous filter in the chain or the AudioClip on the AudioSource. If this is the first filter in the chain and a clip isn't attached to the audio source this filter will be 'played'. That way you can use the filter as the audio clip, procedurally generating audio.