A contract that gives you the right to sell a specific stock at a fixed price (called the strike price) before a certain date. You are not required to sell the stock, only have the option to do so. You pay a fee upfront, called a premium, to buy this contract.
You buy a put option on Microsoft stock with a strike price of $300 that expires in one month, paying $8 per share. If Microsoft's stock drops to $280, you can exercise your right to sell it at $300, which is worth $20 more than the current price. If the stock rises above $300, you simply let the option expire and lose the $8 premium you paid.
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