By "Hidden" I simply mean everything that does not have a dedicated knob (or similar) that makes it obvious what it is, on the equipment being used. Gear manufacturers and software designers choose what to make dedicated controls for. For someone who is "learning by doing", they will try tweaking the controls that are laid out, and try to come up with the best sound. The "hidden" stuff is all the aspects that are hidden in sub-menus, or require several steps (like gain-staging) or are not the kinds of things for which there could easily be a knob.

I mentioned phase, polarity and gain-staging, because for someone just recording track after track on a DAW, tweaking some levels and doing some panning, it's quite possible to make serious errors regarding phase, polarity and gain-staging, without knowing that these errors occurred, or how to avoid them. The mix doesn't sound as good as it could have if those things were known.

Some "hidden" stuff might not be very hidden, they could be there to tweak, but the user might not reach for it because they started out on a system where that function did not have a control.

Another "hidden" thing could be keyboard shortcuts in DAWs that massively simplify some aspect of the work, like quantize to grid, for those that use that. Imagine someone trying to align two tracks by just moving them with the mouse, and then they learn about a keyboard shortcut that just does that, and it makes life so much simpler.

Yet another "hidden" thing is how important the room response is for monitoring. Someone could buy great monitors and even put up some absorption and diffusion and expect great translation, but if they are unaware of Sonarworks (or similar), they will put in a lot of hours into mixing their stuff while still not getting a mix that sounds great elsewhere, if they sit right where waves cancel out each other.

That's what I mean buy a "key insight". Just knowing that room modes can make a massive difference, and acting on that knowledge, could elevate their work to a whole new level. The same can be said about knowing the difference between what happens when an analog signal is pushed to distorsion to tape, vs what happens if a signal is pushed to distorsion inside a DAW. That's a key insight in the sense I mean here. And it's a piece of knowledge that is not obvious by just looking at knobs and meters, since they typically look the same in the DAW as on a mixer going to tape.