You know the sound engineer dudes who make seemingly random shifting movements on those huge banks of levers when making a recording? Well, what’s the correct wording for what they doin’?
I do believe that’s what’s known as “mixing”.
But how would one describe the action, say, if you wanted to communicate what was happening to a child, and you weren’t in the studio so they could see it?
I believe some people refer to those sliding levers as “faders” but I don’t know what the term is for moving them.
Tell the kid that they’re big volume controls like you have on the radio in the car, and he’s adjusting the volume.
The correct term for moving the faders is “pot up” or “pot down” - pot standing for potentiometer - the technical name for “volume control.”
I believe the verb for adjusting a fader is slide.
Sliding a fader.
Playing with his knobs 
If you are just trying to explain it to child or layman, just say that he is adjusting the sound to make it more human (ie life-like). That microphones only hear and allow playback in bland machine speak and that only humans can add real life to the sound. If he watchs any of the newer cartoon shows this day he will understand. Mine did. 
Rico and Wolf are right: the engineer is using the faders to mix the recording.
Generally speaking, each fader controls the the volume (and other controls adjust tone and other audio characteristics) for a different channel. When recording, each instrument is miked with a separate microphone and recorded to a separate channel on a multichannel digital recorder (or tape deck, if you’re very old fashioned).
The tracks are not necessarily all recorded simultaneously. In fact, except for symphonies and such, it’s rather rare. The rhythm section may lay down the basic tracks in the first pass, then that recording is played back and the vocalist records the lead vocal onto a separate track. Then the backup vocalists come in, the horn section, and so on. An ordinary studio session may have 24, 48, or even more channels in the final recording.
In the original recording process, each channel is recorded on its channel at the highest level that will avoid distortion, to provide the highest quality. The final recording we buy on a CD is made by mixing all those channels to appropriate levels, adding effects, like reverb, and turning it all into an artistically pleasant whole.
Modern mixing consoles can automate the mixing process, recording the changes the engineer makes during a song–bringing up an guitar solo, for instance–and recreating them during the final mix. This simplifies what could easily be – with many different adjustments possible to 48 channels – an otherwise impossible task for one person.
I hope this helps.
Good explanation commasense. The automation system can record the Engineer’s exact movements of the faders (more advanced systems could record EQ and panning moves), and the movements can be played back later - even re-recording additional movements during the playback.
In reinforcement (live sound) situations, when the Engineer had to make fader movements in real time, a common word for his actions would be “riding” the faders.