A bid group allows you to added any number of auctions into what is called a "bid group folder". You then tell the folder how many items you want to win. It will keep bidding on those auctions, until it wins the number of auctions you specified, or runs out of auctions. Let's say you want to win 2 items out of 10 auctions - once you've won the 1st 2 items, the bid group folder stops bidding. Does that make sense? Any other questions?
They're very easy to work with.
Here's the bid group tutorial.
http://auctionsniper.com/bidgroup.aspx