We can't see your tax return or your letter. Maybe the IRS thinks you failed to declare some income, or took credits or deductions that they don't think are allowed. You will need to read your letter carefully. You probably only have 30 days to reply with a defense.
Here are some resources to get started.
https://turbotax.intuit.com/tax-tips/irs-letters-and-notices/dont-fear-the-tax-man-handling-irs-lett...
If you paid for Audit Defense when you filed, contact the company directly with the enrollment number you received.
