A FFI is an interface for a language, such that it can call procedures or functions from other languages no matter what language they are programed in.
For example this means given scheme code and a target language.
There are a number of applications of this: Reuse of preexisting libraries and source code are obviously one of the more important aspects. 1) allows to do this by writting code in Scheme, whereas 2) requires code being written in the target language, which can be considerably less comfortable, depending on the language. Also see PPR:ForeignFunctionInterface. An interesting FFI survey is in the context of AutoLisp is Design Issues for Foreign Function Interfaces.
The plt-scheme FFI to C is detailed in Foreign Interface for PLT Scheme.