Originally, I gather, it was a way of making sure that the person named had actually written the thing - hence 'viva' - meeting the living person.
It's when you 'defend' your thesis to a panel of examiners. They ask you questions on it and so on. Best case scenario, they're already happy you've gone about it all correctly and produced the best work you can, and pass you (even better when they tell you at the start). Worst case, they think the whole thing needs redoing. In between, varying degrees of questioning/terror.
I'm really quite glad I won't be going through it now.
Congratulations LR, that's good going. Now, relax!