Ah,,, maybe a twist of terminology.
Mixing..... is where you do your magic. Like the term, wet and dry tracks. A dry track is essentially an un-effected track while a wet track is a track that has had "effect" applied to it. Mixing is when you add effects, set panning, adjust volume or just leave it alone on a per track basis.
Mixing Down (QuickMix) or in the version 1.3.0 I use, Mix And Render.
Is when you put all your tracks together to make 1 file. Mixing down.
Of course, this is the LAST thing you do.
But, during the recording process, and this may help people with latency issues in Audacity, you can "pre-mix" tracks. What I mean by pre-mix is... for example: You may have an acoustic guitar recorded on one track and panned hard right. You may have another acoustic track that is panned hard left. But in the same recording session, you may have a bass track, a vocal track and a harmony track. In order to save space AND IF YOU ARE SATISFIED with your two acoustic guitar tracks, you can highlight both tracks, by holding the SHIFT KEY and clicking on each track. They both are now highlighted. Now you go to the PROJECT tool and do a QUICK MIX on just those two tracks. All other tracks are still in their natural state. Now you have one less track to deal with.
This process saves loads of space and resources on your computer. Plus simplifies things when you get 10,15 or a couple dozen tracks going at one time.
Hope this helps. Again, others more wise than I, have much more concise answers.
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LC