Shorthand and acronyms are persistent on Reddit and some are used more widely in specific subreddits. You'll become familiar with them as you spend time on the site.
Here's a crash course to get you started:
"According to the OP, the image was taken from The New York Times."
"TIL that taxonomically, modern-day birds aren't just descended from dinosaurs, but are considered to BE dinosaurs."
"DAE think that Bert and Ernie from Sesame Street should have their own full-length sitcom?"
"IAmA Police Officer in Arizona, AMA"
"I have had 2 ACL reconstructions. AMA."
"[After a long, detailed story] TL;DR I went to Vegas, found a monkey in my hotel room, and lived 'The Hangover' in real life."
When someone makes a change to another user's post or image, often with humorous results.