This is a "hóngbāo" (红包), a red envelope for giving money. It's an important part of celebrating the Lunar New Year in Chinese-speaking regions, symbolizing good fortune and luck for the new year.

What's important is not the amount of money, but the red paper itself, which is believed to ward off evil spirits. Traditionally used for Lunar New Year, it's now also given for celebrations like weddings and birthdays. With the rise of digital payments, online hóngbāo are also popular.