Definition
The IRS form a U.S. person gives to a business that pays them, so it has their correct taxpayer identification number for reporting the payments.
What it means
Form W-9, Request for Taxpayer Identification Number and Certification, is how a U.S. citizen, resident or U.S. company gives its name, address and taxpayer identification number (a Social Security number, ITIN or EIN) to someone who will pay it, such as a client, a platform or a bank. The payer keeps the form and uses the number on information returns such as Form 1099-NEC or Form 1099-K. The form is not sent to the IRS. If you do not provide a correct number, the payer may have to apply backup withholding to your payments. People who are not U.S. persons use a Form W-8 instead, such as Form W-8BEN for individuals.
Example
A freelance designer starting work with a new U.S. client fills out a W-9 with her Social Security number so the client can report what it paid her at the end of the year.
Illustrative numbers only. Your situation will differ.