TLDR: The point of a TLDR is to say if a person only wants to read the summary, well here it is.
It means "if you couldn't be bothered to read the preceding material because it looked too long (and possibly boring), here is a summary for you". The meaning is quite close to 'executive summary'.
If TLDR is mentioned in the post, the point is to provide a summary of the lengthy text so that someone can skip to the TLDR section and get a quick overview of what the story talks about without having to read the whole thing.
Comments that include the letters "TLDR" usually indicates that the text was too long and they didn't want to read it, but it might instead be the commenter's summary of the content. It might be used to tell the poster and other commenters that the comment might not be reflective of the post since it wasn't read in full, or it might be a little joke to show that this post is way too long and nobody has time to read all of it.
More Information on TLDR Usage
In the first usage mentioned above, when TLDR is in the post, it's a helpful subject line summary, where the poster offers a one-sentence or two-sentence summary of the many paragraphs to follow or precede the post.
TLDR is most commonly seen in very opinionated discussion forums, where the topics lend themselves to long rants. Controversial topics, like Barack Obama's healthcare policies, climate change, immigration, or the ethics of speeding in the city, can easily lure people to write hundreds of words of heated opinion.
However, TLDR posts can really be anywhere, including computer help forums and even online stories.
In the second usage of TLDR, the comment might not be quite an insult but rather a suggestion that the user above should consider abbreviating their writing. This might be used when the previous poster submitted more than a couple of paragraphs in the conversation.
Many people who edit Wikipedia do so because they enjoy writing; however, that passion can result in overlong composition. This reflects a lack of time or commitment to refine an effort through successively more concise drafts. With some application, natural redundancies and digressions can often be eliminated. Recall the venerable paraphrase of Pascal, "I made this so long because I did not have time to make it shorter."1
A second contributing factor can be that a writer incorrectly believes long sentences and big words make them appear learned. Or an inexperienced contributor may fear they will not be clear enough with fewer words. Even capable authors recognize the risk of distorting what they're trying to express in too-brief passages.
Some policies and procedures can encourage overlong prose due to imposing arbitrary limits. The Did you know? process requires established articles to have a fivefold expansion of prose within a seven-day window to be considered for listing on the main page. This can encourage over-verbose writing to game the system.
A trusted aphorism states that "brevity is the soul of wit." Similarly, "omit needless words." Editors are encouraged to write concisely and use plain vocabulary when possible, always keeping in mind English may not be a reader's native tongue. If length is essential, a short summary is advised.
While bloated composition may reflect the emotions of an editor, it should be noted that some people are constitutionally loquacious. It is impossible for you, as an editor, to affect either of these before the fact. When editing, always respect Wikipedia policies and editors' feelings. Take the time to distill your thoughts for better communication and rapport.
A further option for both readers and writers, is to structure the writing so it can be skimmed effectively. This means writing the first sentence of each paragraph as a summary of the paragraph, so the reader can quickly know which paragraphs or sections are of interest to read for more detail, in addition to the usual practice of putting a summary at the beginning of articles or sections. This works even when the content is concise, or for some uses should be complete, but a reader wishes to skim for speed in a disciplined and more accurate way.
Internal policy discussions on talk pages can often become longwinded, too, usually for two reasons: because of the detailed nature of Wikipedia policies and guidelines (and their often complicated interaction with each other), and because curt and questionable assertions of policy rationales (especially when many are made in series in a single post) may require fairly detailed responses. The cure for this problem is to make a clear policy-related statement to begin with, and avoid citing more policy and guideline pages than are necessary to get the point across (many say the same thing in slightly different wording). If you cite five such pages in vague terms for the same point, you open the door to wikilawyering about wording and interpretation – you may get five paragraphs of rebuttal in response, instead of one sentence of agreement.
I don't understand why people would put the TLDR here, as the person has to scroll all the way to the bottom to see it.