When you buy put options you hedge (reducing your risk) in that particular stock. Buying puts gives you the right, but not he obligation, to sell the underlying stock at a certain price. You are paying a premium for this measure of security, but you are also limiting your potential downside.
Example: You own 1,000 shares of Microsoft (MSFT) and you're afraid it may go down in the next month. The stock trades for $30 and you think it may go to below $28. You buy put options with a strike price of $28 for $.50 each, costing you $500.00. If MSFT goes below $28, you can exercise the put options and sell your MSFT at $28. If the stock doesn't go below $28, the put options will expire worthless, and you don't get your $500 back.