What is a RAR!
A RAR! thread refers to a thread with a "highly complicated" hypothetical scenario. Originally it was a purely derogatory remark (and is occasionally still used as such). Now it is a somewhat well known aspect of SD.net Culture and has taken on the role of simply describing a hypothetical scenario that may or may not be stranger than your usual ones. Mere versus threads are not considered RARs, however RARs can include versus elements inside them.
RAR is not an acronym, so stop asking what it stands for. Some people have commented that is stands for "Read And Review." These people are however idiots. Others believe it to be an anthropomorphic sound of a roar. This is likely borne out by the context of Mayabird's post. It is also a type of downloadable file, used for compression or archiving of large items for easier transmission across the Internet.