It's called Proxy Bidding.
Let's say the item starts at $1.00, and you really want it. So you put your max bid in for $10.00, and walk away. Now, the auction will still show that it's $1.00, but it will have a bid on it. Anyone who comes in to bid on that auction will be outbid automatically up to $10.00. This is part of the brilliance of eBay. You can bid your max bid, and walk away. If you win, you win. If you don't, you don't. The best part about proxy bidding is that if you are indeed the only person to bid on said auction, you will only pay $1.00, rather than the $10.00 you bid. The $10.00 was just the max you bid, just in case more people bid on the auction.
Make sense?