Mixing is taking all of the recorded tracks and making them work together. Making sure that each individual track is in its place and audible. Automating certain instruments at certain times to make sure they stand out when they are supposed to. Making sure the drums are hitting as hard as they should, paying attention the the vocals to make sure everything is understood. Always paying attention to the melody. This of course changes drastically from song to song and what style of music it is. Mixing a song is like an opinion, I have an "opinion"  of how I think it should sound and John Doe down the street has a different opinion.  I recommend spending the time to check out engineers discographies and finding someone that you think fits your style or what you want out of a mix.