From Halopedia, the Halo wiki
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TLDR is an achievement in Halo: The Master Chief Collection. It is unlocked by beating the 25 minute par time on The Library. The achievement is worth 10 Gamerscore and can be unlocked on any difficulty.
The achievement's name is a reference to the meme/internet slang "tl;dr", an abbreviation for "too long; didn't read", used to express disdain at an overly long forum post or similar block of text that is deemed overlong and not worth the time it would take to read. In this case, reading arises from the typical use of the word "library".