In the most simple way I can explain, let's say you have an auction that lasts 24hrs from first bid, which was received yesterday at 5PM. That auction will finish at 5PM today, regardless any bids, UNLESS any bids are received after 4:45PM today. If you receive a bid at 4:46PM today, the auction will end at 5:01PM, if someone bids again at 5:00 PM, the auction end will be pushed back again, and will end at 5:15PM, and this will continue to be pushed back like this until a full 15 minutes has passed since the last bid