Online auction sniping is a term that means placing your bid during the final few moments of an auction. This can do one or more of the following things:
1. Prevent another person from reacting and placing a higher bid amount.
2. Prevent "bidding wars" from raising the price of the auction throughout its listing duration on eBay.
3. Keep "Your value" for an item a secret from the rest of the world until you are ready for it to be revealed, which is usually at the end of the auction.
4. Allow you more control over your bids because you they can more easily be deleted or modified in our system before they ever make it to eBay.