Tax withholding is when your employer holds money back from your equity compensation to cover ordinary income taxes. Because your award is considered a form of compensation, your employer is required to withhold taxes. These are the same types of taxes that you see withheld from your paycheck. Depending on your plan type, you may be able to choose how these taxes are paid. Check your plan's documents to see whether this applies to you.
If you're able to choose your withholding method, simply click Select Withholding Method from the Quick Links menu of your plan. From there, follow the prompts to apply your changes. You can also use the Setting Up Your Restricted Stock Tax Withholding (PDF) quick-start guide if you need help along the way.
To learn more about what tax withholding means and how it works, review the Tax Withholding in Company Stock Plans (PDF) guide.