Audio Mixing is a crucial step in the music creation process. Mixing happens after the recording and before the mastering. Mixing requires a great deal of technical and musical skill, focus, and years of practice. In this part of the musical process, an audio engineer takes the raw recorded stems of a track and blends them together into a stereo track that is then ready to be mastered. 
 
The act of mixing involves four basic processes; Level Balance, EQ, Compression, and Effects. 
 
Generally the first step in mixing is to get the levels or volume of every track exactly where you want it, so each layer “sits” in the mix properly. This step is usually roughed in during the recording of the track. 
 
Next is EQ, or equalization. EQ is when you manipulate the frequencies of a sound. Some consumer audio  products like Bluetooth speakers or car radios have knobs or setting that allow you to turn up the bass, mids, or treble/highs. The process of professionally EQing a song takes that idea to the next level. An audio engineer will skillfully map out and carve out the whole 20 thousand hertz spectrum of the human hearing, and perfectly place every frequency to make sure the song is complimented in the best way possible. 
 
Compression is way of controlling the dynamic range of an audio signal. Compression can be used correctively or creatively. The basic idea of a compressor is that it makes the the loud stuff quieter and makes the quiet stuff louder, therefore compressing the sound into a tighter dynamic range. If you have a track that has a lot of loud peaks and a lot of quiet stuff in between and want to even them out that is when a compressor could be used correctively. One example for a creative application is to get the song to “pump”, by having a long release time, the compressor will still be compressing when the next loud hit happens causing a pumping effect that could sound cool. 
 
And finally Effects, everything else can fall under effects. Reverb, delay, distortion, etc. all adds that little extra bit of flavor. And that is only skimming the surface, there’s still panning, comping, tuning, gates, and automation, to name of few more steps of the process. 
 
Once you’re done with the all of that, it can be sent to be mastered which is another art in its own right.