The act of doing a silent laugh, chuckle or snicker. It is usually performed by widening the nasal cavity, and exhaling a large amount of air through that cavity.
It is used as a substitution for "lol", since the majority of internet users do not actually laugh out loud, but rather leedle.
Guy 1: "Oh em gee, dude. My fart sounded like Chuck Norris cracking his knuckles."
Guy 2: "Leedle."