Mastering means different things to different people. The most complete definition of mastering is preparing the final mixes for what will become the final "master recording" from which all subsequent copies will be made. Usually mastering includes:
- Adjusting the start and end of the track to be neat and clean - "tops and tails" as they say.
- Determining the best order that the songs will go in - an order where there will be a natural flow, etc. This flow is also influenced by the length of silences between tracks, etc.
- Adjusting the tonal balance of the tracks so they are consistent from one to the next - so they "sound" like they belong together.
- Creating a "DDP file" to what they call "red book standard", including the various metadata that CD players use to display song titles and artist names, genres, etc., and the UPC and ISRC codes that retailers and broadcasting facilities use to track sales and usage. This is what gets uploaded to the manufacturing plant, and they use this to make their glass master, from which all the manufactured discs are made. (Most of them don't just "burn off a bunch of discs.")
- Adding compression and limiting to the tracks so that they have that final fished "polish" that we associate with professional recordings, and so that they are as loud as other commercially available tracks.
For many people, when they refer to "mastering", they are referring to only the last bullet in the list. Sometimes, like if you are doing one song to be released only in the online domain, this is enough. However, if you are preparing a product for sale, the other bullets need to be given fair consideration. I can help lead you through those steps that need to be taken care of by you (registering songs, etc.) and taking care of the technical aspect of things so you don't have to.