Did you file your tax return by mailing it? Did you forget to sign the return in ink? Did you attach your W-2's, W-2G's, or any 1099's that showed tax withheld when you mailed the tax return?
Form 3531 – This notice is typically issued because the IRS is missing a valid signature or requires proof of federal withholding and needs the information to finish processing your return. Another reason this type of notice is sent is that the social security numbers or legal name of the taxpayer reported on the submitted return does not match information received from the Social Security Administration.
Box 4 states "Attach supporting document(s) for your entry of $ XXX for tax withheld. This may be a form W-2, Form W-2G, form 1099, or substitute form W-2.
So is seems that you did not attach copies of any forms that support the total tax withheld entered on the return with the mailed in return as required so they sent this form 3531 asking for the missing information... simply provide what they want as directed ASAP as the return will be in limbo until you do.