The cIOS a custom version of IOS, which are Input/Output Systems that run on the console's Starlet processor. They are similar to drivers installed a PC which allow the software communicate with the hardware and vice versa. Every so often, these IOSs on your Wii will get updated via Nintendo or game discs to support more hardware or for better compatibility like the USB keyboard. A cIOS allows the Wii to communicate with its hardware in ways it wasn't originally intended to. They are released in the form of installers, which use a base IOS, such as IOS 56 or 57, and add modifications to it then install into an IOS slot thats empty, which typically be an IOS slot above 200.
► D2X cIOS - The recommended and only neccesary cIOS that needs to be installed.
► hermes cIOS - An additional cIOS that can be installed to provided better compatibility with certain niche games.