External
external, or "FFI" (foreign function interface), or simply "interop" (for "interoperability") is how Reason communicates with other languages, like C or JavaScript.
Imagine writing a let binding, but with its body omitted and its (mandatory) type written down:
external myCFunction: int => string = "theCFunction";
Note: externals can only be at the top level, or inside a module definition. You can't declare them in e.g. a function body.
Usage
You'd use an external value/function as if it was a normal let binding.
For more infos on how to link up your C functions within your Reason application, have a look at the Dealing with foreign libraries section in the dune build system docs.