In simpliest way, you and other customers join the same deal, and seller will offer a discounted price to win your business.
It is a well known rule: volume drives price down, and it also works in online shopping
Sellers are competing for the group purchase with best price they can offer
Check the offer's current price and see how many more join to get next tier price
Book your order with shipping address and payment information
Before offer closes, when more customers book orders, your final purchase price may move to the next tier!