Set the EQ flat on your bass and amp. If you normally use an overdriven or “hot” sound, set it to clean.
Play an open G. Play it a bunch. Play it softly, play it hard, stop it short, let it ring out, etc. You can even slap it, if you’re into that.
Now play a G on the 5th fret of the D string.
Now play a G on the 10th fret of the A string.
Finally, play a G on the 15th fret of the E string.
Alternate between different combinations of these G notes. They’re all the same pitch (if they’re not, you need to set your intonation, which is super easy and there’s no excuse not to do it; see the upcoming post on “Setup for People Who Hate To Think About This Shit”). But timbrally, they are vastly different.
The way you choose to describe these differences is subjective, because it depends on your technique, the particular instrument you’re using, how it is set up, the acoustic qualities of your amp, and finally your ears. The one thing that is constant across all instruments is that certain parameters will change in a pretty linear way as you go from the first G to the last G. Come up with your own set of words to describe the difference between the open G and the G on 15th fret of the E string. These differences will be the most obvious. Once you’ve done that, try describing the qualities of G on the 10th fret of the A string and G on the 5th fret of the D string as well.
By now you should be starting to notice things, and feel free to try playing other pitches in different areas. There is no limit to the nuances you can find. Did it take you less than an hour to do these two exercises? Do them again. Do you still not see the point? Do them again. Do you think you get it? Do them again. Pay particular attention to the difference between open and fretted notes, and the regions of the neck that seem distinct from each other timbrally.
This exercise comes with a homework assignment which you have to do every time you pick up your instrument from now on: every time you have to play the G that is around 97.999 Hz, make sure it’s the best one.