The Processing language can be used in electronics applications for interfacing microcontroller circuits to a PC. The computer interface software can be written in Processing and allow a user to interact with the microcontroller circuit or embedded system. The Arduino can be interfaced to a computer in this way and there are many Arduino / Processing PC interfacing examples on the Internet (e.g. the Arduino relay shield project).
The article on the system requirements for the Processing language explains the hardware and software requirements for running Processing.