What is put option in share market?

A put option is a contract. It gives you the right to sell a stock at a price that is fixed. You must do this before a certain date. You buy a put if you think the stock price will go down. It is like an insurance for your stock. If the price does go down, you can sell at the higher, fixed price. But you do not have to sell. It is just your choice, your 'option'.