The lottery which has been going on for at least 77 years as Old Man Warner tells us, is for the purpose of human sacrifice. Each year, in June, the town holds a lottery, a selection process, whereby each family chooses from the black box, a slip of paper. If your family is unlucky enough to select a slip of paper with a black dot, then your family is chosen.
Then your family members, children included must choose from a group of slips of paper to determine which member of the selected family will be sacrificed. The winner of the lottery is stoned to death by the other members of the town who display little or no sympathy, mercy, compassion or interest in the death of one of their neighbors.
What is so striking about this short story is that the author, Shirley Jackson, allows the reader to believe that the lottery is accepted by the townspeople the same as a dance, or picnic. It is a deeply held tradition embedded in the culture of the town.
No one can remember what the original reason for the lottery was, they can't remember the ceremonial songs that were part of the ritual, but they keep holding the lottery anyhow.
Fear that their crops will not be full and hearty is suggested as one reason for the lottery. Some ancient cultures believed that human sacrifice satisfied the gods and therefore, the village would be granted a plentiful harvest in return.
One is sacrificed so that many may live.