Anything that is fun. It can be used as a noun, adjective, or verb if you're feeling adventurous.
When spelled backwards or with an "anti" added to the front it has the opposite meaning.
"Im going to buy some treatsies."
"Want to smoke a treatsie bowl?"
"Im gunna treatsie you until the cows come home!"
"You're the treatsiest person I know!!"
"I love you treatsie."
"Organized religion is an eistaert!"
"No more ganja, anti-treatsie!!"