It means that your bid (your full, maximum bid, even if the full amount is not shown) is lower than the "Reserve" set by the seller as the minimum sale price, and you cannot win that item with that bid.
It means that you can safely move on to another source for a similar item without fear of winning this one, unlike merely being outbid where a cancellation of the higher bid can make you the winner and obligated to pay at the end. If your bid did not meet the Reserve at the time it was placed, nothing anyone else can do can make you obligated to buy that item (even if the seller lowers the reserve, eBay will lower your maximum bid if necessary to keep you from winning; you would have to rebid to have a chance to win this auction).