Shorting can be seen as this: Right now BTC price is $500 a coin (example) and you think it will be $400 in a week. You don't have any Bitcoins to sell yourself right now so you basically "borrow" someone elses 10 bitcoins and sell them for $500 now. You now have $5,000, but you owe that "person" back 10 coins. So as it turns out your right and the price drops to $400, you now buy back the 10 coins you owed at $400 each, so you spend $4000 and you pay that person's coins back. You just made $1,000 off a falling bitcoin price.

I'm sure someone that uses bitfinex or any other stock exchange often can explain it far better than I, but that's a start. So a high amount of short orders right now, means that many many people are feeling the BTC price will continue to fall....and they are putting their money where there mouth is.