If you're old enough to remember, that was the name of a song from the band R.E.M. in the mid 1990s. What's the frequency is also a question that a lot of sound engineers and musicians struggle with.
One of the things that can wreak havoc with a live performance is feedback. I'm sure many of you can relate... you are struggling to get the vocal over the insanely loud stage volume and dodging feedback bullets left and right. While you search frantically with the EQ to figure out which frequency it is, the audience and band is glaring at you. Not a pleasant situation to be in.
When you can’t identify frequencies it can be very difficult to get things to sound the way you want them to.
For example, how to make muddy sounding vocals really shine and pop in the mix. How to mix so you can hear all of the instruments without them interfering with each other. How to make the PA system to work with the room acoustics.