Long Put strategy means to Buy Put option. Long Put is opposite of Buying Call. In Buy Call strategy, you are bullish about the stock/index. But when investors are bearish, they can buy a Put Option.
The Put Option give a right to sell stocks at a specified price and thereby limit his risk.
A long put is a bearish strategy to take advantage of a falling market. The risk is limited and rewards are unlimited.
A bearish investor can profit from declining stock price by buying Puts. He limits his risk to the amount of premium paid, but his profit potential remains unlimited. This is one of the widely used strategies when an investor is bearish.