I read some guy describe some music ( I think it was Dethklok) as having 'implied melody' and for some reason it made perfect sense at the time, but I still can't really grasp it. Can anyone make sense of this idea or explain it?
If you saw it on Metalocalypse it was probably just a joke. It's most likely Brendan Smalls take on the pragmatism of some bands. Don't worry, he probably had no idea either.

But, there is a such thing as implied melody. If you analyse a melody and only look at the most important notes (Highest/lowest notes in a phrase, loudest notes, rhythmically accentuated notes, longer notes and chord tones and so on) and take out all the embellishments, you could form another melody inside the old melody.

To explain this better: read this http://profile.ultimate-guitar.com/demonofthenight/blog/52428/ and do the opposite.
[U]        | |                     [/U]
[U]        |/     .-.              [/U]
[U]       /|_     `-’       |      [/U]
[U]      //| \      |       |      [/U]
[U]     | \|_ |     |     .-|      [/U]
      *-|-*    (_)     `-’
Essentially, it's the basic form of any melody. Take a guitar solo and take out every note except the really important ones. The simplified melody is the implied melody.