Have you ever posted an auction at a price per stack when you thought you were pricing it per item?
What about selling an item for 500,000g, but you forget a few zeroes, and instead post it for 500g?
What happened as soon as you posted your auction? Someone running a sniping addon probably snatched your auction away before you could cancel it. Thousands upon thousands of gold lost.
This addon attempts to prevent that from happening. Every auction you try to post will get checked against a pricing source (like The Undermine Journal, TradeSkillMaster, Auctionator, or others). If you try to post an item for under 20% of its market rate (or, in the case of TUJ, under 2 standard deviations of the average), Boomerang will block it and your auction will never go live. You'll see a message in the chat window of what happened so you can evaluate and fix your post.
In case you double-check your price and you're sure you want it that low, you can toggle the addon with a quick /boomerang and it'll let all future auctions through (though it will still warn you in chat).
Let me reiterate: you also need a pricing addon for this to work. Most folks already run TSM and they'll be covered. TUJ and Auctionator work, too.
Why call it "Boomerang"? It's an anti-sniper device. Obviously.