Silent describes the bidding style: private bids rather than an auctioneer. Online describes where bidding happens. Most silent auctions today are both.
The minimum an item must reach before it sells. It's useful for items that cost you something, such as consignment items, but it can stall bidding on everything else.
The minimum amount each new bid must add to the current highest bid. Set it high enough to move bidding along without putting bidders off.