I Use Bitcoin t-shirt

What the heck is Bitcoin?

"Bitcoin is a peer-to-peer currency. Peer-to-peer means that no central authority issues new money or tracks transactions. These tasks are managed collectively by the network." - Bitcoin.org

So, it's like digital cash without the banks and regulations of other currency like GBP or USD?

Exactly, and you can download the official client from bitcoin.org.

And then I buy this t-shirt to help promote the currency revolution that is Bitcoin?

Right again! If you wish, of course. :)