There can sometimes be a bit of mystique surrounding mixing. But it boils down to it referring to the process of putting multiple layers of audio together to make one final and homogenous track. This track becomes, or at least is supposed to become more than just the sum of these multiple layers.
The next question that needs to be answered is what constitutes a good mix. Doing so is never easy. Mixing can be a personal or individualistic activity. It is therefore, nothing less than an artform as far as sound engineers are concerned. That is why if five different engineers or producers attempt mix the same song, chances are, you will have five different sounding mixes.
Bektas (2014), however, lays out seven different characteristics that apply to what a good mix is.
Each track, be it a vocal, voice over, sound effect or music instrument must be distinguishable from other tracks. For example, a kick drum must sound distinct from the other parts of the drum kit. A lead vocal track is usually the focal point of any song and should not be competing with background vocals or guitars in a song.
Aside from being clearly heard, a good mix must provide a sense of space. It should allow you to perceive how some sounds are coming from the left or right, as well as from the back or right in front.
There has to be a good proportion of sounds on the left and right sides. But on top of that, there also needs to be spectral balance, meaning there is a pleasant sounding distribution of bass, midrange and treble. I fall short of saying that there has to be even distribution with both left-right and spectral balance because that would not hold true every time. Balance is still relative to the product, be it a song, an voice over, or a dialog in a movie.
A hook or chorus in a song, or the building of tension in a movie — these are things which the author of the material is supposed to deal with. A good mix is not really expected to do the work for them, but it can help in reinforcing them.
Finishing touches on an audio project is done during mastering. But again, it is a stage for minor enhancing and polishing. Mastering is not meant to fix major issues. A project must already sound good by the time mixing is finished.
An allowance is typically made where the final mix does not peak above -4dB. This gives the mastering engineer leeway by making it possible to apply equalization and additional effects without introducing distortion or making it necessary to compressing or hard limiting the mix and compromise the dynamics
Bektas, M. (2014). Audio Effects, Mixing and Mastering (Kindle version). Retrieved from http://www.amazon.com/Audio-Effects-Mixing-Mastering-Bektas-ebook/dp/B00KECZLG2.
Murphy, R.C. (2012). Audio Recording Boot Camp (Kindle version). Retrieved from http://www.amazon.com/Audio-Recording-Ronan-Chris-Murphy-ebook/dp/B00785UE0G/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1456197409&sr=1-1&keywords=Audio+Recording+Boot+Camp.
Tromp, H. (2011). A basic guide to mixing and mastering. Retrieved from http://music.tutsplus.com/tutorials/a-basic-guide-to-mixing-and-mastering–audio-11038.