An option is a finantial instrument that allows you to chose wether you buy (a call option) or sell (a put option) an specific good or intrument in a specifictime in the future, at a specific price.
When talking about an option, you can choose if you are going to exercise your right to buy (if it is a call option) or sell (if it is a put option).
This is a difference between a future and an option: a future is a compromise to buy or sell an specific commodity or finantial instrument, while the option makes it optional (to buy or sell).
A stock exchange is a simple exchange of stocks, but without a compromise to do it at a specific time in the future or prices. There is no option or obligation to buy or sell in a common stock change case.
In this case, we are talking about a put option (an option to sell), and the finantial instrument that is linked to the option are shares of stocks.
Then, in a specified time in the future you will be able to sell shares of stock using a put option.
