This is essentially the balance between the tracks, think about backing vocals during a guitar solo? Or a calming acoustic guitar under a softer voice and song. The volumes here would be very different. It can be changed using tools called compressors, which control the dynamics of the tune.
This is the location in which the instrument is placed within the stereo field. It can range from hard left to hard right or anywhere along that line. If they were all in the centre, you wouldn’t be able to tell which was which and the song would sound really cluttered, so spread them out to get a clearer mix.
This can change the focus from one frequency to another, be it a melody or a vocal. So if you boost a vocal frequency to high it will add air to them, if you focus on the bass it was make it sound more booming, and if you put a guitar in the centre range it may smooth out the sounds for a chilled out tune.
To effectively mix you need to know how to insert effects, most of which are time based. There are others that can change the sounds of instruments in order to create a different ambience in the song. This is where it gets a lot more complicated but we’re sticking to how to mix and master for dummies, so more detail in that after you’ve mastered the basics.