A ForeignFunctionInterface (FFI) is an interface that allows calling code written in one programming language, from another that is neither a superset nor a subset.
The term comes from CommonLisp; though it's applicable to any such interface. Other language communities don't use the term FFI as much; some languages such as Java have their own term -- Java refers to it as the JavaNativeInterface.
The most common cases are interfaces that allow calling CeeLanguage or CeePlusPlus from a higher-level language.
...with the caveat that the collection of functions that are callable in CeePlusPlus is restricted -- generally things that are callable from C.
Implementing an FFI can be done in several ways:
Requiring the called functions in the target language implement a specific protocol. JNI works in this fashion; any C/C++ function called by JNI must be defined in terms of a specific set of Java-compatible datatypes. Converting this stuff to standard C datatypes and back is (or at least was) the responsibility of the called native code. JNI also provides ways for C code to call back Java code -- necessary in many cases, especially if the C code needs to "hold on to" Java objects. The Java GarbageCollector does not examine the native code for references, so C code that holds on to Java objects must explicitly mark them reserved/unreserved (this is not necessary for objects which are arguments to native functions, but whose native functions go out of scope when the function returns).
Requiring functions declared native to use a subset of high-level functionality (which is compatible with the low-level language).
See also "Design Issues for Foreign Function Interfaces", at <http://xarch.tu-graz.ac.at/autocad/lisp/ffis.html>.
If we refer to the low-level language as the "host" language and the high-level language as the "guest" language, then these three approaches boil down to ways to bridge the gap between them, as follows.
The host is expected to bridge the gap. You have to write host-language functions specifically to be called from the guest. An API may be offered for the host language to use to communicate with guests.
The gap is bridged by some kind of tool that does not belong strictly to either the host or guest languages.
The guest is expected to bridge the gap. This means the guest language can call any host-language function, but the guest language now has to have support for many low-level features, in order to communicate with the host language effectively.
Like almost all modern languages, VbClassic can call C code. VbClassic can also use COM objects, Microsofts component technology, which uses the *other* method of FFI, which is requiring all participants to implement a specific calling convention and data definition used for communication.