Juice is a porting "ecosystem" that allows iOS apps that are written in Objective C and C/C++ using the Apple tools and libraries to be rebuilt to run on non-Apple platforms. It is a "middleware" layer of tools and libraries sitting on top of the Marmalade SDK.
Juice allows you to recompile your iOS code for other platforms rather than having to rewrite your code. It produces native instructions for the target platform, so avoids any overhead of runtime interpretation. It does not automatically alter the source code; the original Objective-C/C++ code remains unchanged.
Where is the documentation?
The Juice documentation is currently stored in a separate space here: Marmalade Juice.