too long; didn't read

That's the literal meaning. Subtexts include: this is way too verbose for the information it's delivering, this is way too editorialised, this needs editing, etc. So "tl;dr" is also a judgement of the resource referred to.

http://encyclopediadramatica.com/Tl%3Bdr#Chapter_1

Thanks.

Note that this actually originates from the SomethingAwful Forums.

Or rather, http://www.google.com/search?q=%22tl%3Bdr%22

More specifically, a summary of a longish text: item 2 from http://en.wiktionary.org/wiki/TLDR