This section is about Audio Modulation. One example of this is Amplitude Modulation (AM). When you modulate the volume of audio with a LFO you get a vibrato effect.
This section is about MultiBand processing. MultiBand processing is about splitting an audio signal into several frequency regions and applying separate processing to each.
This section is all about dynamics. The dynamics of an audio signal is about the range in level of the signal: from the softest whisper to the loudest explosion.
This section is about the automatic envelope. We create an envelope that follows the level of an audio signal. This automatic envelope is sent as a CV signal to control various parameters in Reason.