TL;DR is an abbreviation for "too long; didn't read" that is used to indicate that the person posting about an article or other type of content either didn't read the text it in its entirety or didn't read it at all.
TL;DR is often seen on social media sites, where it usually accompanies a link, and perhaps a comment, based on the title of the article or something the poster has heard about it. The abbreviated term is also used as a comment on verbose postings on blogs and discussion forums.
TD;LR is also seen all lower-case as tl;dr. The correctly-applied semi-colon in TD;LR makes it unusual as most abbreviations do not include punctuation. As a full sentence, TL;DR might be: It was too long; I didn't read it. Some people also include the period when they use the abbreviation; others omit punctuation at all and type it as TDLR.
TD;LR is more precisely known as an initialism, which is the term for an abbreviation made up of the first letter of each word in a phrase. Initialisms are distinct from acronyms, which, strictly speaking, are abbreviations (including initialisms) that can be pronounced as words.