I see your account is 1 day old, and your question makes total sense to me too.

So, welcome, glad to have you here of course.

The answer is a commented "tldr" is widely used on news.ycombinator.com, and, as comments below say, it's become a conventional courtesy: people tend to scan comments for tldrs, in order to possibly avoid digressing into commonly (but not always) long winded posts.