Wiktionary
tl;dr
init. (alternative form of TL;DR English)
Wikipedia
TL;DR
TL;DR, short for "too long; didn't read", is Internet slang to say that some text being replied to has been ignored or as a signifier for a summary of an online post or news article.
The abbreviation is based on the principle that, if the writer does not invest the time to convey their message concisely, the reader is justified not investing the time to read it. Alternately, it might mean that there is insufficient material of value or interest to justify the time required to read it.
The phrase dates back to at least 2003, and was added to the Oxford Dictionaries Online in 2013.