According to Wikipedia, Auction Sniping is:

Auction sniping is the process of watching a timed online auction (such as on eBay), and placing a winning bid at the last possible moment (often seconds before the end of the auction), giving the other bidders no time to outbid the sniper. Some bidders do this manually, and others use software designed for the purpose. A bid sniper is a person or software agent who performs auction sniping.

In domaining, the auctions usually refer to domain drop auctions like SnapNames, NameJet or Godaddy.