I have no idea what this means...
Is this text language or something?
It means Too long; didn't read.
Let's say, you wrote a passage about something and, oh, the horror, it's longer than Twitter-approved 140 symbols of a verbal sneeze.
Here comes some entitled moron and finds your text. For his everlasting shock, it's longer than a tweet. And, oh, the horror, he has to commit to reading it. And put an effort in it - oh, the tragedy. It takes time in which he could watch reality shows - injustice! And some words are longer than 8 characters - violation of his human rights! And his untreated ADHD and attention span of whole 5 nanoseconds does not even allow him to go through the text - absolute disgrace!
So he musters up all the arrogant ignorance he has and blurts out - "TL;DR" - "Too long; didn't read. Tee-hee. You are a dumbass, you know. Who writes something that long these days. But like whatever. I'll go and watch Gossip Girl and then post new photos of my nail polish to Instagram."
On the other hand, I've never seen a truly interesting, compelling, smart and eloquent person use this phrase. They are mostly always engaged and if they are not you'll never know - they just move on quietly.