sorry for my ignorance. what is the difference between mastering and mixing?
No need to apologise, there was a time when none of us knew what mastering was.
Mastering is balancing and enhancing the final stereo mix of your track. If your song is produced and mixed perfectly, you may not even need mastering, but generally, a good mastering engineer will identify frequencies that could be enhanced or are overpowering, add a little compression perhaps, and find a good dynamic balance for your songs.
Mastering engineers tend to work on collections of songs (albums) making sure they form a coherent listening experience. Also, they don't work with near field monitors like the ones your song is mixed on, so they have a different perspective too. A well mastered song will have a layer of 'polish' that's generally missing from the final mix.
I'm sure if you ask nicely here or do an internet search you'll be able to listen to some before and after examples.
Hope that helps.