So... like... what happened, exactly?

It sounds astonishing, but I can't make heads or tails of it. Rosemberg and his team managed to add new letters ("letters") to DNA encoding, and that allows them to make new proteins... how? Did they make new kinds of acid bases for these proteins – the kinds that don't exist in nature? Is that not an even more astonishing achievement?

I'm way out of my depth here, but I'm also intensely curious about anything related to genetic engineering. Could someone explain this to me?