An audit is a systematic, independent examination of whether a management system meets the requirements — and whether it is complied with in practice. Internal audits are performed by the organisation itself; external audits by customers (2nd party) or certification bodies (3rd party).
A good audit compares three layers: what the standard requires, what the procedures say, and what actually happens on the floor. Deviations between the layers are the findings — and the most valuable findings are those the organisation can learn from, not those that can be explained away.
Note the difference between an auditor and an accountant: an auditor examines management systems (e.g. ISO 9001); an accountant signs off on financial statements.