A put option is the right to sell a stock, at a price, for a period of time.

A long put strategy has limited [defined] risk and unlimited potential gains from moves down in the underlying price. 

A long put by itself is a bearish strategy. One way to apply it would be to buy it below major resistance such as a bid/ask spread. 

However, the underlying security (such as a stock) must outperform the premium placed on that put by the options market. In other words, a long put must beat the options market in order to profit. The long put is bought with the expectation that it will outperform the price of its premium, which is what the options market is pricing in for the most likely bearish movement.