Originally referring to a specific joke (where a leaf has no money for a taxi), the term "leaf joke" now applies to any lame joke that you and your friends tell to outsiders, laugh at, and pretend is funny.
This results in a lot of confusion by the outsider, and they'll think they've missed the point... and typically laugh so they look like they "get it".
The Leaf Joke (ending):... and the dim sim said "No"
Your friends: HAHAHAHAHAHAHAHAHAHAHAHAHA!
Outsider: (what??) Oh! Yeah! hahaha!!!