Fu dao is a rather simple visual and verbal pun that plays an important role in the Chinese New Year.
The pun involves the idea of fu (which could be translated as "happiness" or "prosperity"), and the hopes that "fu" will arrive in the New Year. The word for arrive in Chinese is "dao", which is identical in pronunciation and almost identical in the written Hanzi to the word for upside down. Therefore, people communicate their hopes that happiness will arrive in the coming year by taking the character for "happiness" and hanging it upside down.
It is not perhaps the most sophisticated or meaningful pun, but it is an important tradition in Chinese culture, and around the Lunar New Year, many foot tall renditions of the character for happiness can be seen hung upside down.