If your TIN (tax identification number) is incorrect or not certified, 28% instead of 30% backup withholding will be withheld."
It took a while, but I finally worked it out. As a stand-along notice, it's nonsensical: do something bad, and we'll withhold less of your money from your payment. Some penalty.
To get what they intended you to from it, you must compare this to the check stub where it tells you that if your TIN is incorrect, they'll withhold 30%. The yellow slip means to correct that to 28%. However, the writer of the yellow slip must have tunnel-visioned on the notice, and didn't foresee the way the sentence would read out of context. Careless writing....