If someone manages to steal your social security number, they often try to file a tax return claiming refunds. To prevent this, you either have to file before them or obtain an identity PIN from the IRS on the IRS.gov website.
Here is the link for the ID PIN, and here is the IRS explanation:
An IP PIN is a six-digit number assigned to eligible taxpayers that helps prevent the misuse of their Social Security number on fraudulent federal income tax returns. Call the IRS at 800-908-4490 for specialized assistance,
Please see our prior post, How to stay safe after the Equifax data breach, to learn more about credit freezes and other protections. And let me know if we can help!