Sellers can choose to add a reserve price when listing an item in an eBay auction. Unless they state their reserve price in the listing, you won't know what it is until you either meet it or bid above it.
If you bid below the reserve price, you'll see a "Reserve not met" message. This means that even if you're the highest bidder at the end of the auction, you won't win the item.
Sellers can lower their reserve price during the auction or make a Second Chance Offer once it ends. We'll send you a message if either of these happen, so you have another chance to buy the item.