To protect you from identity theft, your tax return is currently being reviewed. To help us process your return more quickly, verify your identity and tax return information. If you recently received a letter from us, follow the instructions on the letter. Please have your tax return (Form 1040 series) available and read the website or letter before starting the verification process.
The identity verification process can delay your refund for a number of weeks, so do not expect quick results. Respond promptly when you get the letter.
Sometimes the IRS chooses returns randomly to request verification of identity and sometimes there is something on the return that triggers the request. No matter why the IRS has required you to verify your identity, you need to follow their instructions in order to receive your refund. If you do not verify your identity you will not get your refund.
https://www.irs.gov/identity-theft-fraud-scams/identity-verification-for-irs-letter-recipients