Complex sentence: A mix engineer, also refferred to as "mixer", is the person who, once the instruments, vocals, etc, are recorded, does what's called the "final mix" or "mixdown" of the song.
Split into simpler sentences: A mix engineer, also refferred to as "mixer", is the person who, once the instruments, vocals, etc, are recorded. He then does what's called the "final mix" or "mixdown" of the song.