So, you heard of IOSes and cIOSes. IOS stands for Input/Output System and cIOS stands for custom Input/Output System. The only difference between the two is that IOSes are made by Nintendo and cIOSes are made by coders. Thats all there is to it. cIOSes are made by extracting IOS and patching them with custom code. So, in essence, both do the same job, namely speaking to the system hardware and saying that the specific function the game requests is available or not.

But why use cIOSes? You know that a specific IOS adresses a specific hardware component. You also know that you can't play film DVD's with your Wii, while say, a PS2 can. The problem lies here by IOS, not by the DVD player. The DVD player inside your Wii is capable of playing them, Nintendo just doesn't want you watching films on your Wii. That's where cIOSes come into play. A cIOS does what an IOS cannot do.

To answer the question, yes. It is safe to install cIOSes on your Wii. This is because cIOSes don't overwrite IOS nor delete them, so there is no chance of bricking your Wii. Just don't delete random IOSes, as a noob you don't know what IOS does what. To be honest, even the coders are unaware what some IOSes do. In any case, stay off system IOSes like IOS30,50,60,70 and IOS80.

Most cIOSes install itself in slot 249/250. You also have 222/223 and 224, commonly used for USB loading. Since Nintendo isn't stupid nor ignorant about these coders they launched an offensive. With system updates like 4.0+ they installed what we call 'stubs' in slot 222/223/224/249 and 250. These stubs prohibit the installation of cIOSes.

Uninstalling is just as easy as installing them. There are two ways to uninstall them.
1)If you have the original WAD you can uninstall them via the WAD manager.
2)Use DOP-Mii or Multi-Mod Manager to delete any IOS above the 200.
A warning: NEVER EVER uninstall IOS with a number beneath 100. Those are the slots that Nintendo installs its system IOSes in. Don't blame me for bricked Wii's when you uninstalled IOS between 1 and 100, I warned you.