Monitor microphone levels and positions so performances sound right.
A Mix Engineer combines multiple recorded pieces into a single track of increased depth and volume. So you might take the recordings done individually of the flutes, brass, and woods, and combine them to make the orchestral track of Beethoven’s Fifth. And then you might overlay the voice of a Rapper on top of that orchestral Beethoven’s fifth and create a chart topper.
You spend your days as a Mix Engineer on a mixing board, which is really just a big awesome toy. With it you can blend multiple tracks, adjust their frequency contents, and determine a lot of other properties like pan positioning and instrumental volume. Your goal as the Mix Engineer is to make the finished track balanced, beautiful, and acoustically clear.
You can work wherever recorded sounded is needed, so you might be mixing a jazz band ensemble or a horror movie. After you are done, you pass your work on to a Mastering Engineer who prepares it for distribution. On a film set, this position is referred to as a Re-Recording Mixer.