An Option -One of The Derivative

An option is a contract to buy or sell a specific financial product officially known as the option's underlying instrument or underlying interest.It has an expiration date. When an option expires, it no longer has value and no longer exists.
Options come in two varieties, calls and puts, and you can buy or sell either type.
Buying and Selling
If you buy a call, you have the right to buy the underlying instrument at the strike price on or before the expiration date. If you buy a put, you have the right to sell the underlying instrument on or before expiration. In either case, as the option holder, you also have the right to sell the option to another buyer during its term or to let it expire worthless.

When you sell a call as an opening transaction, you're obligated to sell the underlying interest at the strike price, if you're assigned. When you sell a put as an opening transaction, you're obligated to buy the underlying interest, if assigned.
At a Premium
When you buy an option, the purchase price is called the premium. If you sell, the premium is the amount you receive. The premium isn't fixed and changes constantly - so the premium you pay today is likely to be higher or lower than the premium yesterday or tomorrow.