According to book definition Stock option is:
A privilege, sold by one party to another, that gives the buyer the right, but not the obligation, to buy (call) or sell (put) a stock at an agreed-upon price within a certain period or on a specific date.
I found quite a simplified article on stock options written from the perspective of a layman.
The price the company sets on the stock (called the grant or strike price) is discounted and is usually the market price of the stock at the time the employee is given the options. Since those options cannot be exercised for some time, the hope is that the price of the shares will go up so that selling them later at a higher market price will yield a profit. You can see, then, that unless the company goes out of business or doesn’t perform well, offering stock options is a good way to motivate workers to accept jobs and stay on. Those stock options promise potential cash or stock in addition to salary.