What does the message "You have been outbid" or the email "You are no longer the highest bidder" mean?

If you have placed a maximum bid on a lot and receive this message:

This notification informs you that another bid has surpassed your current bid. You are therefore no longer the highest bidder and can place a new bid. Please note that this information only relates to bids placed through LOT-TISSIMO. The auction house may have received further bids through other channels.