Withholding is an estimate of the taxes which will be due on the related income. (In this case, your retirement benefits.)
Depending on other factors in the return - such as your other income or deductions and credits - they may or may not be sufficient to cover your tax bill. This may affect your refund or balance due.
If you end up owing, you may wish to change (increase) your withholding if you expect the circumstances to continue.
