What happens after you talk to the man in Jubilife city for mystery gifts on Diamond?

In Pokémon Diamond, after talking to the man who gives you Mystery Gifts in Jubilife City, he will give you an item called the Mystery Gift Voucher. This item allows you to receive Mystery Gift items over the internet. You can do this by going to the Communication tab in the X menu and selecting "Receive Gift." You will then be able to enter the serial number or password for the Mystery Gift item that you want to receive.