Mastering is the last final touch to make your song the best possible version it can be. It could be considered as a frosting on a cake - if a cake itself is poor, it won't become good no matter how great frosting is. However, if the cake is delicious, frosting only needs to emphasize the best qualities of the cake and maybe add a touch of some other sensations.

Mastering focuses on giving the song final polish and gloss, while ensuring it does compete with today's standards (unless artist specifically wants something unique).

WHAT DOES MASTERING REALLY MEAN?