Sometimes, to help reduce the tax burden on stock options, a company will make it possible for option holders to early exercise (or forward exercise) their options, which means they can exercise even before they vest. The option holder becomes a stockholder sooner, after which the vesting applies to actual stock rather than options. This will have tax implications.
caution However, the company has the right to repurchase the unvested shares, at the price paid or at the fair market value of the shares (whichever is lower), if a person quits working for the company. The company will typically repurchase the unvested shares should the person leave the company before the stock they’ve purchased vests.