I called the IRS today they said I am under review and a letter was sent out. What does this mean?

Here is information provided by the IRS regarding general timelines:

If you provide the information the IRS requested, the IRS should correct your account and resolve the refund issue (generally within 60 days). 

If you file a missing or late return, the IRS will process your returns and issue your refunds (generally within 90 days).

If you don’t provide the information or file the missing returns, your refund will be delayed longer.

If you file an amended tax return (IRS Form 1040X, Amended U.S. Individual Income Tax Return), the IRS should make any necessary adjustments and issue the refund (generally within 120 days).

If you’re facing serious financial difficulties and need your refund immediately, contact the Taxpayer Advocate Service.  It may be able to expedite your refund.

For more information, review the information on this IRS advocate website:  https://taxpayeradvocate.irs.gov/get-help/held-or-stopped-refunds