An audit letter in the mailbox is one of the more visceral experiences in American adult life, but most audits are routine civil matters about whether the math on a return holds up. Roughly three-quarters of IRS examinations are correspondence audits — done entirely by mail, no in-person meeting. The rest are office audits at an IRS location or, much more rarely, field audits at your home or business.
The Taxpayer Bill of Rights at § 7803(a)(3) guarantees the basics: you have the right to know why the IRS is asking, the right to bring an enrolled agent, CPA, or tax attorney, and the right to appeal anything they decide. An audit does not mean you did anything wrong. Returns get pulled for random selection, statistical scoring (the IRS's DIF score), 1099/W-2 mismatches, or because something on a related return triggered a flag.