It's about controlling the sounds and their volumes so that each one compliments the other.
Tensions in the track are controlled with dynamics and EQ so that all the elements work as one.
When this doesn't happen, it's easy to get into an area where you're 'chasing faders'.
This is when you can't get the balance between the sounds and you turn up one instrument, only to find out it's drowning out another.
So then you turn that one up and the next and the next, until you've worked your way across the whole desk! 'Chasing faders' can happen with new engineers or when there's too many people involved in the mix.