What is Frequency Masking?
Also called: masking
Frequency masking is when a louder sound makes a quieter sound in a similar frequency range hard or impossible to hear. In a mix, it is the main reason instruments get buried even when their faders are up.
Our hearing has limited resolution: when two sounds share the same frequencies at the same time, the louder one dominates. Kick drums and bass lines, guitars and vocals, and pads and keys are classic pairs that mask each other.
Turning the buried part up often makes the mix louder without making it clearer. Better fixes are to cut the overlapping range in one of the parts with EQ, pan the parts apart, use sidechain compression so one ducks for the other, or change the arrangement so they do not play in the same register at the same time.
A spectrogram can help you see where two parts pile up, but whether the masking is a problem is decided by listening.
Put it to work
Generate a spectrogram of your mix to see where the frequencies pile up.
Generate Spectrogram