Leaving a Bid means entering a proxy bid, so the system will bid on your behalf without the need for you to watch the auction. Simply enter your maximum bid and the system will bid for you during the auction up to your maximum bid.
You need to register on Register To Bid Online and be logged into your account, to have the option of leaving a proxy bid when viewing an item.
If you are the winning bidder then you will pay the final selling price which may be lower than your maximum bid.
Register to bid here