WiringPi is a library that allows easy, Arduino-like access to hardware functionality (GPIO pins mostly) of the Raspberry Pi minicomputer. That library is written in C so what this package does is to provide a simple interface to use the C library from OCaml. The API is unchanged (even the capitalization) to provide an interface which is as similar as possible to the code that it is based upon.
Please note that you need to install the WiringPi library first, as it is not included in this package. Otherwise compilation will fail.