What is Mixing: is a question prone to getting big scary answers with lots of layers of technicalities and pseudo-rules.
Very simply mixing in music is the act of Balancing the parts of the Arrangement so that they best portray the Story of the material.
As an example, if we take a song with drums, bass, guitars, piano and a singer: the most important part of that Balance has to be the lyrics that the singer is delivering (or why have a singer at all). The words and their delivery are the key to the song. Sure, people will seem to react to the drums & bass or a chunky guitar riff, but words and the message they emote, are the secret sauce of any track.
Melody is next. We are driven to singable tunes so keep melodic parts clear in the mix. Guitars & piano can be rhythmic, melodic or a bit of both. Decide which part each line or section is and mix accordingly.
Rhythm often seems the only reason people want to make music but it is the main reason that musos get stuck in their compositions and then in getting exposure.
Your Drums and Bass are there to enhance the natural rhythm of the lyric & melodies. If you put them on top of everything then you may think the mix is great (lead Kick Drum anyone lol) but the listener will not be so impressed as the Balance of the parts is wrong.
As a guide, the style of the material will influence the sort of Balance you look for.
A Folk Ballad will be very strong on Vocal & Melody but light on drums etc. A Rock Ballad will be similar but rhythmic elements will be stronger, but never so as to obscure the Lyrics.
A Disco song will push rhythmic elements forward but not to the exclusion of the lyric and melodies as even though the dancers seem keyed to the beat, they are really driven by the emotion of the mood of the vision of the song. An Indie Shoegaze track often buries the vocal a bit inside of swirling chordal movement but after a listen or two the lyric should still be clear.
Metal is mixed a lot like Disco!