Hedging is a way of reducing some of the risk involved in holding an investment. There are many different risks against which one can hedge and many different methods of hedging. When someone mentions hedging, think of insurance. A hedge is just a way of insuring an investment against risk.
Consider a simple (perhaps the simplest) case. Much of the risk in holding any particular stock is market risk; i.e. if the market falls sharply, chances are that any particular stock will fall too. So if you own a stock with good prospects but you think the stock market in general is overpriced, you may be well advised to hedge your position.