The human hearing range is often referred to as 20 Hz to 20 kHz (20,000 Hz), but this is just some nice round numbers that happen to be reasonably close to average. In reality, most people don’t hear this full range, and some rare folks can hear beyond it.
Every octave is double the frequency. So, if you play the A string on your bass, it’s going to be 55 Hz. Go up an octave (2nd fret on the highest string) and it’s 110 Hz. Go up another octave, and it’s 220 Hz. Go up one more octave and you are at the A that is above middle C on the piano, which is 440 Hz, which, coincidentally, is the standard we use for music these days. You can google “440 Hz” for more info on how this came about.
If you pluck the A string, it is going to vibrate at 55 Hz. But, if you were to look at it through a slow motion camera, you’d find that it’s not a perfect vibration, especially since you pluck it from near one end of the string. Instead, you’ll find that there are all kinds of little harmonic oscillations going on up and down the string. These higher frequency components are what makes each instrument sound unique. Woodwinds have very few higher frequency components to the sound waves they make. The sound out of a woodwind is almost a perfect sine wave. A brass instrument is at the other end of the spectrum. Brass instruments have huge amounts of higher frequency components to their sound. A bass guitar isn’t going to have a lot of high frequency components to its sound, but it will have some.
If you use your EQ to filter off all the high frequency sounds, what you’ll find is that you mostly hear only the main frequency that each string is vibrating at (called the fundamental frequency). This is often desired when the type of music has a lot of other higher frequency instruments, and the bass is there just to fill out the low end of the sound. You’ll also note that most of the “plucking” sound gets filtered off too. The bass is left as more of a background instrument. If you want the bass to be featured more, or you want it to fill in some of the sound, then you will want to include more higher frequencies on the EQ. The most extreme example I can think of for this would be the rock band Rush, who uses the bass guitar not only for the bass notes of the songs, but also as a countermelody to the guitar, so in their case it is important that the bass stand out, and not blend into the background. No matter what style you use, there’s not much on the really high end of the sound spectrum from a bass guitar, so you can often kill off the really high frequency bands on the EQ to help get rid of noise.
An EQ allows you to have a lot more control than just low and high frequencies. For example, you can filter off the lows and highs both, and just keep the mid range frequencies. This will give your bass a sound that isn’t booming loud on the low end, and blends in fairly well with other mid range instruments, and yet isn’t so punchy that you have to worry about it intefering with the sounds of guitars and other higher frequency instruments. Think of the warm blended sounds of the 80’s. These days, most bands prefer a more open and aggressive sound, which you get by keeping more low end and high end frequncies, but cutting back a bit on the midrange frequencies.
If your bass has two pickups, then you’ll want to adjust the volume controls on each one in addition to adjusting the EQ. The pickup closer to the middle of the string tends to get more of the fundamental tones of the strings. The pickup near the bridge gets more of the higher frequency tones. You wouldn’t want to use the rear pickup (near the bridge) and then filter off all of the high frequencies in the EQ, since all that would remain are the lower frequencies which don’t get picked up so well by that pickup, for example.