UC Gift Cards are what we'd call gift vouchers or certificates in the UK. It's a version of money which can only be spent in certain shops. UC's usage is pretty wide though, so if you have one, read on to see what to do with it.
This kind of card is distributed by many companies or institutions as a prize, compensation or refund. You can also buy them.
There's a site in Japanese with full details, and here are the basics in English (for the first time online! Another AccessJ first, watch out other sites!).
Half a million stores and hotels across Japan accept these vouchers, including (I bolded some big names I know of):
UC also does various other services, such as credit cards, ETC cards and all manner of other bits and pieces best tackled only if you can read Japanese to a high standard (see the site). You'd be better off with our guides on credit card applications and getting an ETC card.