This is an acronym for ‘Too Long; Didn’t Read.’ It can appear as TLDR, tldr, TL:DR, tl;dr.
Online writers sometimes use it to provide a summary at the beginning of their own article. A long-form description of a sporting event may have something like this at the top: TL;DR: my team won a tough match.
It does make communication faster in informal emails and posts, but it can sound rude or sarcastic if not used carefully. The Urban Dictionary added it as a word in 2003.
This TL;DR summarizes the plot of a comic (2.34 mins).