The Mixing Engineer's Secret Weapon
Whether you're learning to record, working live audio, or final mixing a session, one of the absolute essential techniques to learn is how to use a high-pass/low-cut filter and it's slightly less-used counterpart, the low-pass/high-cut filter. Every DAW and every EQ plugin has one (I personally recommend FabFilter's Pro-Q3).
However, a lot of aspiring audio engineers miss this one really simple technique to clean up their sound. Instead they might start adding compression (enhancing unwanted frequencies), additive equalization (EQ curves that add gain), and side-chaining (triggering off unwanted frequencies) to their tracks before they've cleaned out the unwanted frequencies.
Pro-tip: Pass filters and subtractive EQ are some of the most useful and overlooked tools that can immediately kick your mix up a notch or 2 without doing anything else. They can quickly fix frequency issues like muddy midranges and bass frequency conflicts (kick and bass sharing the same Hz space). They can also create separation and create EQ "pockets" to let things like vocals sit properly in the mix.
There are so many great tutorials online to learn this technique from. Here are some we've liked:
6 Ways to Use a High Pass Filter
What's a High-Pass Filter & How Should You Use It?
7 Tips for Using Subtractive EQ
A High Five for the Low-Pass Filter