The sound made by someone or something eating.
Most commonly found in lolcats
and similar macros
. It usually translate as, "I am absorbed in eating this".
The phrase may relate to the sound Cookie Monster makes on Sesame Street when eating cookies.
Also "Om nom nom".
*picture of kitten asleep in food bowl*
"OM NOM NOM"
Joe: So how's that burrito?
Jack: Om nom nom nom!
Joe: So good, then?