TL;DR, short for "too long; didn't read", is Internet slang to say that some text being replied to has been ignored due to its length. It is also used as a signifier for a summary of an online post or news article.
The phrase dates back to at least 2003, and was added to the Oxford Dictionaries Online in 2013.
On reddit, tldr is the subreddit serving as the site's daily recap. Long posts on reddit are also frequently followed or preceded by a summary, consisting of a sentence or brief paragraph, that is prefixed by the phrase "TL;DR", and the abbreviation is sometimes used as a noun denoting such a summary, as in "Please include a TL;DR along with your post".
The phrase inspired the name of Terms of Service; Didn't Read, a website that aims to analyze and grade the terms of service and privacy policies of major service providers.
The reddit bot "autotldr", created in 2011 automatically summarizes news articles in the comment-section of reddit posts. It was found to be very useful by the reddit community which upvoted its summaries hundreds of thousands of times.
Various applications, services, websites, groups and individuals that summarize lengthy content exist online. Wikipedia and especially its lead sections can be seen as such a website.
Furthermore some journalists have started adding "tl;dr"s to their articles and many already add summarizing subheaders. The uses of such summaries include that readers can thereby decide whether or not they are interested in investing their time in reading the full article.