Ducking is just reducing the channel volume dynamically. For example when someone isn't speaking.

A Side Chain controls some effect on one channel based on the signal from a separate channel. Such as "ducking" background music when someone is talking.

Ducking specifically refers to lowering the volume of one track based on peak levels in another track. One track "ducks" under the other. It's related to a side chain input on a compressor.

Dynamically lowering a quiet signal is done by an "expander", or a "gate" if it's lowered to zero.

Listened for a few minutes: I'd tweak the EQ so that voices sound more pleasant. In fact the remote voice (is it Zoom or something similar?) sounds awful to be honest, but that's the shortcoming of this type of recordings. So you need to apply a different EQ to the remote voice: reduce the highs and slightly boost the mids/lows, but it may end up being tricker than that.

Ducking in general is an effect used a lot on radio: you have a background music that automatically "ducks" when there's a voice over it. In other words, background music gives way to the voice.

Thanks. It was Skype, actually.

What a stupid comment. Are you just on the lookout for something to be offended about?

I was asking for an explanation, which he gave.