Consider IOS like the Wii main programs. It contains all instructions for games and console system menu to work with the hardware (NAND access, USB, SD, etc.)
There are different IOS with different set of functions: Some are used for music games to enable access of USB to use microphones and instruments, other IOS may contains Wifi modules, etc.
Each IOS are installed in different Slot number, and loaded when needed (only 1 at a time). Possible slots are from 1 to 255, though current official IOS are installed into the 80 first slots only.
To enable USB access in games instead of using the DVD reader, you will need to modify these IOS files.
Custom IOS (cIOS) contains instruction for accessing USB2.0 HDD, and many more things that official IOS files can't do.
cIOS are installed in slot > 200
There are different cIOS too, each of them doing something different. The main cIOS are : Hermes, Waninkoko, d2x
The "d2x cIOS" is the most complete and the most up to date.
So, you need to install the cIOS required by the homebrew you want to use : USBloader require HDD USB access.
cIOS are installed when following the ModMii instructions to hack your Wii.
Once you've followed ModMii, you don't need to worry about cIOS, they will be all installed as they are "required".
About forwarders:
You have different ways to launch a program/homebrew.
- SD card, from the Homebrew channel.
- Wifi (through another receiving homebrew on the Wii)
- USB (through a special development device, USBgecko)
- Channel: Installed directly on the Wii memory, listed in the Wii menu as a channel. It contains the full program in the channel (like WiiWare games).
- Forwarders channel: Forwarders are Channels installed on the Wii menu, though these kind of channel doesn't contain any real program (no USBloader, no game, etc.). The content of the Forwarder is only a program which load another homebrew located on the SD card or USB drive. It forward to another place. The interest is that you don't have to update/reinstall this channel on the real program update. Just change the program file (boot.dol) located on the SD card and the homebrew is updated.