Some auctions have a reserve price which is a hidden minimum selling price, on their item. The reserve price is the minimum price at which the seller will sell the auction item for.
When posting an auction, a seller sets a starting price. This amount is usually low, allowing the seller to attract bidders who then drive up the price naturally, by placing bids.
The seller can also choose to set a reserve price. This price is higher than the starting price. If the reserve price isn't met by the close of the auction, the item won't be sold. Neither the bidder nor the seller will be under obligation to complete the sale, and no contact information will be exchanged.
If an auction has a reserve price that hasn't yet been met, this information will be displayed as 'Reserve not met' next to Current Bid in the item page.
The seller usually specifies a reserve price if they are not sure of the real value of an item and wants to reserve the right to not sell if the market value is less than expected.
When you're bidding in a Reserve Price Auction, bid as usual entering the maximum amount you're willing to pay for the item. Watch the label next to the current price to see whether the reserve price has been met. Until you see that the reserve price has been met, there have been no successful bids in the auction. Once the reserve has been met, the item will sell to the highest bidder when the auction closes.
If your maximum bid is the first to meet or exceed the reserve price, the effective bid displayed will automatically be adjusted to the reserve price.