When you receive something of value from your employer, it generally counts as taxable income. This includes grants of stock or other perks such as a housing allowance or a car.
Like most Bay Area tech workers, I'm compensated partially in stock. Every month, X shares of my restricted stock "vest", which means, in practical terms, that I receive those shares. That counts as taxable income at the current market value of those shares. To avoid making me pay a large tax bill come April, my employer actually withholds some of those shares of stock to cover the tax bill.
(I've never been entirely sure how this stock-withholding works from a tax perspective. Do they immediately sell those shares and pay the proceeds to the government along with the rest of my income tax withholding?)