TLDR - also known as Too Long; Didn't Read.
TLDR can be used in one of two ways. First, it can be used as a way to mark a summary for a very long blog, forum, or other community post. For example, if someone were to post a thread containing several very lengthy paragraphs and wanted to make certain his or her main point would not get lost in all of the speculation, evidence, research, etc. that came along with it, he or she might add TLDR (or some variation thereof) at the end of the post, followed by a sentence or two summarizing the point he or she was trying to get across. In this case, TLDR can be looked at as shorthand for, 'here's the idea I'm trying to get across'.
The second way one can use TLDR is in response to a very long post. In this case, TLDR is usually (but not always) used caustically, and is generally telling the writer of the very long post, 'hey, there are way too many paragraphs here for me to focus on the main idea'. It is generally considered impolite to post 'TLDR' in response to a lengthy post.
TLDR is a term generally found in blogs, forums, chatrooms, and other such community sites; it is rarely seen in fanfiction, though the odd TLDR-troll does occasionally pop up in the comments section.