This is an external interface to the type translation facility. In the implementation, all foreign functions are ultimately defined as type translation wrappers around primitive foreign function invocations.
This function is available mostly for inspection of the type translation process, and possibly optimization of special cases of your foreign function calls.
Its behavior is better described under
CFFI-USER> (convert-to-foreign "a boat" :string) => #<FOREIGN-ADDRESS #x097ACDC0> => T CFFI-USER> (convert-from-foreign * :string) => "a boat"