Metroid Prime 3: Corruption

you have to send the friend voucher to your friends on your wii's friend list who also have Metroid Prime 3 and that will give them a green credit and if any on sends you a friend voucher you can get a credit. These mostly help unlock the last of the concept art and the whole bonus gallery

"Tell me everything I need to know"

Friend vouchers are earned gradually through the course of the game (the easiest ones to get are just kill count, so kill everything). They can be sent to friends, and your friends can turn them into Green Credits. In addition, friends can send you vouchers that you can redeem for credits. You and your friends must be on each other's Wii Friends lists, you must all have a Corruption save file, and have the game configured to interact with WiiConnect24 (which also of course needs to be enabled). This can be done in the game's options area.

"How do you send?"

In Prime 3, go to the friend voucher area, and it will show your Wii Friends list. Anyone on the list who is capable of receiving/sending vouchers with you will be colored white. Simply select their name and confirm with the game that you wish to send them a voucher.