"TL;DR" means "Too Long; Didn't Read." It's used to tell people that something is too long or complicated to read, or that they don't have enough time or interest to read it all. It can also refer to a short summary of a longer piece of content to make it easier to understand.
• TL;DR: Profits down 20%.
• My teacher assigned us a really long chapter to read, but I didn't have enough time, so I just read the TL;DR summary online.
• I summarized the key points in a TL;DR at the top.