The Reserve Price allows the seller to set a minimum price at which they are willing to sell the item being auctioned. Many sellers start auctions at a low price to encourage bidding, but set a Reserve Price to keep the auction from selling for too little.
In the event that an auction ends without meeting the Reserve Price, the seller and bidder are not obligated to complete the transaction with each other. The seller may negotiate with the winning bidder for an acceptable price, or they may cancel the sale and relist the auction.
In the event that the Reserve Price is met, or there is No Reserve on an auction, both the seller and the buyer are legally obligated to continue with the transaction as normal.