As PMRants said (from what I remember) Melodic Death Metal is when they take the chaotic instrumentals and/or vocals (basically like regular death metal) and add harmonious melodies to it. Which it's completely badass.

A couple of examples

Paul Wardingham GHOST IN THE MACHINE: https://www.youtube.com/watch?v=bIwjJPpFbCU

Scar Symmetry GHOST PROTOTYPE 2 DEUS EX MACHINA:
https://www.youtube.com/watch?v=Z5aB9bW5B00