No, it's not that the characters run around with no clothes on. That would be the Hentai edition. This is simply a pared-down version of Big Eyes, Small Mouth Fourth Edition. The two games are distinct from one-another. So, if you want the "full game," you go with BESM regular. If you want something that's more rules-lite, you'll go for BESM Naked.
From the post:
Introducing BESM Naked
A Stripped-Down Version of BESM Fourth Edition
It’s time we revealed a little secret about the our upcoming Big Eyes, Small Mouth Fourth Edition launch: BESM 4 is actually two distinct games. In addition to the standard Fourth Edition, we are also releasing a stripped-down core game called BESM Naked that a complete, stand-alone, alternate edition.
BESM Naked will be presented as a “gateway product” of a sorts, targeted at anime and manga fans who may know something about role-playing but aren’t traditional gamers. It’s also for role-playing enthusiasts who enjoy indie-style RPGs with simple rules and more narrative game flow. With a structural simplicity closer to BESM First Edition from 1997 — yet presented with an eye on illustration and graphic design that lives up to the expectations of today’s gamers — BESM Naked contains everything you need to play, wrapped in one small volume at an attractive price.
And as you can tell from the cover illustrations of the two games, we had some play-on-words fun with the titles!
With a unique, simultaneous approach to publishing two new RPG core books in a way that’s never been tried before, you might ask us “Why are you doing this?”
• The standard BESM Fourth Edition is an excellent game that reflects the pinnacle evolution of its Tri-Stat System, though some players still prefer the simple elegance of BESM First Edition. Not everyone wants comprehensive
rules with robust options; some people just want to play an easier storytelling game and enjoy a more-descriptive and less-mechanistic system.
• Although BESM Naked doesn’t contain all the specific details and options of BESM 4e, the two core rulebook are highly compatible and integrate together comfortably.
• We know that many anime and manga fans like to role-play (and cosplay) at conventions, and they may be inclined to play more often if a game was released that matched their interests and expectations. BESM Naked presents an intuitive rule system that’s easy to understand, and consequently it might be the exact product that turns otaku into gamers.