Perhaps one or more of your funders wants you to have an audit or your board has simply decided that an annual audit is needed. The Yellow Book is the U.S. General Accounting Office GAO collection of Government Auditing Standards. It is published with a yellow cover. An audit tells you what the opinion letter states. These are carefully crafted and written letters.
Every word is there for a reason.
Generally the auditor is expressing the opinion that the financial statements and accompanying notes are a reasonable representation of the transactions and account balances of the organization. The auditor arrived at this opinion by selecting samples of transactions and balances to test for completeness and accuracy. The most common are lack of segregation of duties and lack of properly documented review and oversight functions. Generally, the by-laws of the organization will specify reporting requirements. The Finance Committee of the Board may designate itself to conduct an investigation and employ the Finance Director to carry out specific procedures.