From the project’s site:

JSCocoa bridges Cocoa to JavascriptCore (WebKit’s JS engine). It allows you to call C code, ObjC code, use C structs, and build Javascript classes inheriting from ObjC classes.

Basically, JSCocoa allows you to use Cocoa classes and the runtime, only using Javascript. An important note for those following this blog closely, JSCocoa does not perform the same mission or tasks as the two other popular “javascript frameworks” aiming to bring Cocoa to the web today, Cappuccino and SproutCore. JSCocoa would allow a developer to write a full Cocoa application only using Javascript. I’m not really sure why anyone would do this, but it’s nice to have the capability.

JSCocoa Homepage