FFI is a means of calling C functions from other languages.
Support for FFI originated with Rebol2's paid version, "Rebol/Command":
https://www.rebol.com/docs/library.html
It wasn't part of the Rebol3 open-source release, but Shixin Zeng of Atronix Engineering implemented a version of it, described in this 2019 video:
That implementation was baked in to the core of Atronix/R3.
While FFI was not considered a high priority for Ren-C, removing it as being required from the build was a priority. But rather than deleting it, it was migrated into an extension:
https://github.com/metaeducation/rebol-ffi
The extension was put on hold in 2021, but brought back to life in 2025.