A protective put involves buying a downside put option (i.e., one with a lower strike price than the current market price of the underlying asset). The put gives you the right (but not the obligation) to sell the underlying stock at the strike price before it expires. So, if you own XYZ stock from $100 and want to hedge against a 10% loss, you can buy the 90-strike put. This way, if the stock were to drop all the way to, say $50, you would still be able to sell your XYZ shares at $90.