Well, it's hard to describe but it's an interactive stage show ...
The premise is that you are among a group of new members joining a 1930s explorer's club. The "show" takes place during the evening in different areas of the club - the mask room, the treasure room, the main room with the bar, and the library where they have live musical performances. The original club members wander about during the evening and draw audience members into the action. It's a lot of word-play and deadpan type stuff ... people seem to either love it or hate it.
Jay wore his Tobasco polo there the last time we went and the club president christened him "Festive Print-Shirt Boy" for the rest of the night so every time an actual cast member saw him that's what they called him.
You get to take the oath of membership and learn the secret handshake. Kungaloosh is both the signature drink of the club as well as the greeting of club members.
It really is a ton of fun if you like that kind of thing. If you guys get a chance to check it out next month you really should - it's a truly unique experience and it's a real shame that WDW is going to dump it.
EDITED TO ADD: You don't have to stay all evening to be part of the action or to understand what's happening. It's all very fluid. And I'm pretty sure that kids are allowed from 7PM to at least 9PM, and maybe as late as 10PM.