Imagine writing a let binding, but with its body omitted and its (mandatory) type written down:
external myCFunction: int => string = "theCFunction";
externals can only be at the top level, or inside a module definition. You can't declare them in e.g. a function body.
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.