Used in reply to a post and/or comment or other written material that dragged on way too long about things that weren't very important.
Also sometimes used by trolls on posts that are longer than their attention span, or in an argument or flame war when one person can't come up with a comeback.
Also usable on your own lengthy post at the end followed by a quick summary of the above material as a courtesy to people who might not want to read the entire thing. Essentially the same as writing "In Summary," when used this way
1. Guy #1: Did you read that book for english class yet? I'm still on page 537.
Guy #2: tl;dr. The book is mostly made of random extraneous details. I bought the study notes instead, they were only 50 pages long.
2. Troll: lulz ur such a fag
SomeGuy1: It really is pathetic that you would make accusations about my sexuality instead of coming up with a legitimate argument. I wonder if you even watched the video before posting.
Troll: tl;dr LOL
SomeGuy1: How old are you, 5?