|Synonyms:||teal deer, tealdeer, tldr|
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tl;dr is an acronym for "too long; didn't read" or, as a warning, "too long, don't read."
tl;dr is not applied to long posts indiscriminately, but only to lengthy, impassioned screeds, which the speaker literally has not read, or thinks are too in-depth or boring to contribute to the conversation at hand. If it's longer than it is interesting, it is a case of tl;dr.
Posting only "tl;dr" in response to a thoughtful, worthwhile meta post could be interpreted as trolling, but since what makes a post worthwhile is in the eye of the beholder, so is the value of tl;dr.
An alternate form, "teal deer," is tl;dr spelled out as it would be pronounced aloud. (This species of vibrantly-colored wildlife may also be found in icons of a humorous nature.)
Tl;dr can also be used in a self-deprecating manner,  presumably as an attempt to pre-emptively warn readers that something they are about to read may be overly long, and probably not particularly interesting. Fans may refer to their own in-depth analysis as tealdeery.
A shortened summary (either by others or by the original poster) of a long post is sometimes referred to as the tl;dr version.