A night before the beginning of the new year, the children in every household is given money in a red pouch or paper. This is supposed to ward off evil.
"Fu" is written on the paper squares, which can be pasted normally or upside down. In Chinese the "reversed fu" is homophobic with "fu dao le", which means "fu comes". The upside down "fu" becomes a pun that implies "good fortune has arrived". Thus, the paper squares represent the "arrival" of spring n the "coming" of prosperous year.
Nowadays, 'Fu', literally meaning auspiciousness, blessing or happiness, usually appears as a cultural symbol to express people's wishes for the coming new year. Yet, in the past, the character mainly meant luck n fortune, which also represented the unanimous hope of the society...