Backup withholding tax means that you had some investment income that might have been subject to tax. Your broker, however, didn't know your tax status. You might not be subject to this tax. Or you might. They don't know. So they withheld the amount the law requires them to withhold when they don't know your tax status.
When you file your taxes for this year, you'll pay the actual amount of tax due and receive a credit for the amount withheld. If too much was withheld, you'll get some back. If not enough was withheld, you'll owe some more.
You can avoid this happening in the future if you inform your broker of your correct tax status using one of the forms they suggested.
You can find a lot more information in the "Backup Withholding" section of the W-9 instructions:
What is backup withholding?
Persons making certain payments to you
must under certain conditions withhold and pay to the IRS 24% of such
payments. This is called “backup withholding.” Payments that may be
subject to backup withholding include interest, tax-exempt interest,
dividends, broker and barter exchange transactions, rents, royalties,
nonemployee pay, payments made in settlement of payment card and
third party network transactions, and certain payments from fishing boat
operators. Real estate transactions are not subject to backup
You will not be subject to backup withholding on payments you
receive if you give the requester your correct TIN, make the proper
certifications, and report all your taxable interest and dividends on your