This is not really an audit at all. If your notice letter has CP-2000 written on the heading or at the bottom of the page is simply a request for information.
Most of the time these letters are asking about income you forgot to report on your tax return. These letters often include bank interest, securities transactions, W-2 income, or other income items which the IRS has reason to believe should have been reported on your tax return. Sometimes this income was actually reported on the return but the IRS was unable to match it up with records on their end.