It installs a bunch of patched IOSs on your system.
This lets you run Backups from the disc channel.
Though some people don't like having all those IOSs patched.
cIOSCORP patches every ios with a cios. I wouldn't use it though because later on, it can fuck up your Wii really good. I would recommend only installing Hermes and Waninkoko's cIOS.
as mentioned earlier in the thread, cioscrop is now darkcrop - rotation in this manner is actually good for the wii scene, as it keeps the soil of development fresh and new.