Mastering is a process that’s separate from production, as well as mixing; it involves preparing your song for distribution. Mastering requires a unique set of skills, and equipment, which is why people often choose to outsource a mastering engineer. Typically, a producer writes a song, and then sends it off to a mixing engineer who ensures all the elements of the song sound good together. From there, the mixing engineer sends the song off to a mastering engineer who applies buss processing (if required) and formats the song based on how it’s being distributed (CD, vinyl, streaming, etc.).