Mixing is done first for each song on an album.
You mix to make all the parts blend together and fit nicely into their own space.
Once that’s done.
You take all your songs and master them. This make an album sound like it goes together. It makes all the songs sound like they belong together. They will be a similar volume, dynamic (loud parts vs quiet parts), and color.
If you only have 1 song and you mix and master it, all you’re doing is making the song parts (guitar, drums, vocals, ect.) sound balanced with each other and then making it louder by mastering and adjusting its dynamic range with compression. (The quiet parts vs the loud parts and how much room is between them).
There’s a lot more to explain but for eli5 this is already probably eli15.
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