Audio mixing is the stage of music production of doing all that stuff and sorting things out the way you like it e.g. proper gain staging or even drum replacement where necessary.
Sometimes it's just a matter of turning up the gain or mixer fader of the drums if they aren't loud enough when an audio mastering engineer says so, and in some cases, it's about EQ'ing the drums and bass to achieve a punchy sound (for example being more biased towards the rhythm section) but you can't do that when mastering music since you are dealing with a stereo mixdown.
Any boost or cut you make on a stereo file will change the tone of surrounding sounds.