Hi, I am a Mandarin Chinese teacher form eChineseOnline.

The character “福 (fú)” is used in “福气 (fúqì: good fortune) and 幸福 (xìngfú: happiness).” It the one of the most  popular Chinese characters in China. During Spring Festival, Chinese people often buy “福” posters to decorate their doors, windows or walls. By displaying these decorations they are praying that  good fortune and happiness will come to their families during the coming new year. 

If you observe carefully, you will see that the character “福” is always posted upside down.
Chinese people call it a “倒福 (dàofú: an upside down fu).” This is because in Chinese , “倒 (dào)” is a homophone of the character “到 (dào: to come/to arrive)” people believe  fortune will come quickly if they  put the character “福” upside down on their doors. If you see an upside down fu, do not reverse it, because if you do the fortune will flee away.