The process of MIXING is where the artist, producer, engineer works with the individual elements of your song:
1. The vocals – their tuning and sonic quality.
2. The instruments – if stems are present, individually these need to be tweaked so they cater to enhance your song and not just be an instrumental/beat that a producer composed without regard to the song you eventually created over it. Of course instrumentals/beats that are provided as what is called a “two-track” (just a stereo file) can also be sweetened massaged to cater to your song, albeit with obvious limitations.
3. If recording live instruments, much of the same tweaks and evaluations also need to be considered like tightening up performances, adding effects (reverbs, delays, filters, phasers, chops, stutters, breakdowns) – whether subtle or something super creative that really enhances your song.