It has two uses, depending on whether it's "Too Long, Didn't Read" or "Too Long, Don't Read." The first was the original, I think, meaning that someone else's post was long and rambling and not worth reading.
The second is for your own post, acknowledging that what you just wrote (or are just about to write) was long, rambling, and usually whiny (or sometimes just long and informative about something you suspect no one else cares about). Sort of a "Don't mind me, I just had to get that out." It's kind of a pre-emptive defense against anyone else telling you that your post is long and boring.
no subject
The second is for your own post, acknowledging that what you just wrote (or are just about to write) was long, rambling, and usually whiny (or sometimes just long and informative about something you suspect no one else cares about). Sort of a "Don't mind me, I just had to get that out." It's kind of a pre-emptive defense against anyone else telling you that your post is long and boring.
no subject