A share option is the right to buy a certain number of shares at a fixed price, sometime in the future, within a company. These shares offer your employer a way to reward you in addition to your basic pay. The value of the reward is the market value of the shares at the date they were awarded.
You can find further information on shares and your tax obligations in the Additional incomes section.
Any tax due on the exercise, assignment or release of the option is chargeable under self-assessment. Your employer will not deduct tax on your behalf through payroll.