On the first Sunday of most months Latter-day Saints go without food and drink for two consecutive meals, roughly twenty-four hours, and give the church at least the money those meals would have cost. The two halves are deliberately joined. Fasting on its own is an act of devotion that benefits nobody else, and the movement's teaching, drawn from Isaiah 58, is that the fast God chose was to deal bread to the hungry. The offering is what turns an abstinence into a transfer.
The money does not go into the general funds of the church. A fast offering is administered by the bishop of the ward that collected it, and he spends it in his own area, on rent for a family facing eviction, on a grocery order, on a utility bill or a medical account, for members and for others. He is instructed to be generous and to keep the recipient's circumstances confidential, and only the surplus after local need is met passes upward to be used elsewhere. In a poor congregation the offerings of the ward may be less than its needs, and the difference is made up from the surplus of others, which makes the practice a quiet mechanism of redistribution across a global church.
Nobody is required to fast. The sick, the pregnant, the nursing, the very young and anyone for whom it would be unwise are exempt without asking permission, and members are encouraged to give a fast offering whether or not they fasted.