To elaborate one step further. Most instruments have frequencies that are not part of their principal sound. When listening alone you might notice that the guitars sound best Without removing these freqencoes, because having a full spectrum makes them sound full. However, when combining with other instruments, these same frequencies get in the way of their fundamental frequencies. When mixing together, overlapping frequencies can make an instrument sound muddy. Bass frequencies are the biggest culprits but the idea can be spread throughout the frequency spectrum.
Here’s a little trick you can use to hear this in action as well as using for finding where to low cut the bass (track you need to be most careful with low end cuts. Cut too much and it doesn’t even sound much like a bass anymore, too little and the kick drum and bass step all over each other. Solo your drum track and bass track. Put a high pass filter or an eq with a low end cut on the bass. Now start at the lowest frequency possible and sweep to a higher frequency. Keep in mind that you are ‘removing’ volume (in the low frequencies) but you will notice at some point when the low cut makes room for the kick drum, the bass starts to sound crisper and clearer, even louder. Back off a little from there and that’s a good place to cut the low end from the bass. Now you should be able to solo just the bass track and not hear much of a difference in the bass with or without the cut. Be careful not to cut too high (frequency) or the bass will sound hollow. Also keep in mind that for some music with really low sub bass, you may not want to cut the lows off the bass. In those cases sidechaining compressors to duck bass out of the way of the kick is a good option.
Sidechaining is also a way to get more clarity without increasing volume. For example you can use this method to lower the volume slightly for midrange instruments that compete with the vocals. Another example is a horn line. I have one piece that has some horns the overlap the vocals. So by using a compressor side chained to the vocal track, when the vocal is present the horns are a little quieter, then as the vocals trail off, the compression stops, and the horns gradually (play with release time) rose to fill the empty space. Make sure when doing this to only aim for 1-2db cuts at most or it will make it pump too much.
To add one more thing: in a given project there can be tracks that aren’t meant to be heard, but add dimension. It’s natural to put these tracks too loud because we can’t hear them in the mix. When you have tracks that are meant to disappear in the mix, try quickly toggling solo or mute on these tracks (on to off them back on again quickly). By hearing the track alone for an instant or visa versa not hearing it for an instant l, allows one to focus in on that track to hear how it sits in the mix, even though listening to the mix alone, it disappears.