The ENVFOLLOWER / EXT.IN module brings external sound sources in your AE modular; it does this in different aspects:

The module has two mono audio inputs for external sources; one unbalanced for line level signals like audio players, tapes, portable recorders, and a balanced one for directly attaching microphones (dynamic and self powered condenser mics, but not phantom powered ones). This input signal is brought then to AE level.

Now, from the input signal an envelope is derived, giving the volume of the signal as CV which can control what you want, e.g. filter cutoff, pitch or what else is CV controllable within your AE system. The SMOOTH knob controls the shape of the envelope / suppression of the audio signal component.

Finally, the ENVFOLLOWER module provides a gate signal, indicating that the level of the audio input is above a certain level that can be set with the GATE THRES knob.