When You bid on expired domain and You haven't been outbid after end of auction, then You're winner.
Winner means You need to pay for domain You've won and then wait 7 days. This 7 days period brings previous owner chance to renew that domain.
If this happens, You'll get refund. But this doesn't happen that often as if someone didn't read expired domains warnings he'd probably won't check it in those 7 days.