Wow, that's an old rule. I could have sworn that was overturned ages ago, lmao
Anyways, it means that posts cannot be less than four words and/or 25 characters total. I'm not sure if there's still a section that enforces it around here, but that rule's pretty much been abolished and is no longer a forum-wide rule.
EDIT: I see it's still in effect in the Round Table. So I guess that's where you saw it?