With the help of mixing your song will be able to reach a higher, comercial-like level by applying processing such as EQ, compression and saturation to your individual tracks. This will result in things such punchier drums, more pristine vocals or bigger guitars.
Mastering
When mastering, the engineer will make sure that the mix will hit the correct levels before uploading it to medias such as Spotify or Apple Music.
The mastering engineer will also in many cases apply additional processing to the mix, giving it a final quality push.
