What is this RESERVED thing?

Reservation of the top comment (for visibility) for later edits without disturbing the original post. Typically.

RESERVED

I never understood the point when you can just edit the OP. If something is important and you want it near the top of the page so people see it, it should be in the OP. Some people will read the OP, but not check the comments, even if it's the first one.

I could see needing to reserve the top comment if you plan on creating a massive thread with the OP going over the character limit. I think the limit is 10,000 characters.

Better to have it and not need than to need it and not have it. :)