TL;DR: This one pretty much speaks for itself once you know what it stands for: Too Long; Didn't Read.
Seen often on Reddit and other message boards and online communities, it indicates a summary of a preceding text.
In other words, if you see a long thread posting that you don't feel like reading,
or that you would like to know ahead of time whether it's going to be worth reading,
look for a TL;DR toward the end (often in bold) where you'll be able to get the point by reading a condensed version.
Very useful feature.