An audit is an evaluation or examination of something by a person or group of people. They can be made to a person, to a company, to buildings, to systems, to documents and many other things that are used by people.
Audits are made to check something, like a person is paying their taxes correctly or that a document is correct.
The most common type of audit are the audits performed on companies and their financial statements, a type of document. In business, many companies inform and report their operations, their money, and their income in financial statements to many people for many reasons. They inform people like banks, governments, investors, and the public, for reasons like getting a loan, paying taxes, getting investments, and improving their public image.