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".
"OM NOM NOM"
Joe: So how's that burrito?
Jack: Om nom nom nom!
Joe: So good, then?
"Om nom nom nom" is an onomatopoeia