What is "TL;DR"?
TL;DR
Too long; didn't read.
What the f... heck?
"TL;DR" is a tongue-in-cheek way some online users have of saying they refuse to read some long piece of textual content. Sometimes as an insult that it's not worth reading. Sometimes just to point out that there has got to be a shorter way of saying something.
Many websites started playing on this idea by incorporating a shorter-than-short summary of a document and heading it "TL;DR". As though to tease the reader in anticipation of them quipping that the text is too long to be worth reading. A good TL;DR section will actually contain useful information. Often using a funny turn of phrase. But also it's meant to be a somewhat unsatisfying answer because the reader realizes it's actually too short to be of much use. Hopefully they get a chuckle and move on to read at least some of the rest of your document.