RSUs (Restricted Stock Units) are taxed as income when they vest.
Typically shares are withheld to pay the tax before you ever see them. I.e. if your tax rate is 50% and you're getting an award of 100 shares, you'll only receive 50 shares in your account.
After that the shares are treated as if you'd purchased them at market rate. If you sell them immediately, there's no capital gain and no further tax to pay.