Generally, an audit means an official examination and evaluation of documents, especially accounts, by an independent person.
In the cryptocurrency ecosystem, we use a Smart contract audit term. The auditing of smart contracts can be divided into manual and automated. Manual auditing is the process of an auditor examining each line of code to identify vulnerabilities. Automated auditing is performed using automated testing programs such as Truffle. So to summarize what smart contract auditing is: Smart contract audit is a code review by auditors to determine if the code is secure if there are any vulnerabilities, coding errors, or logic errors that could have critical consequences for the protocol and smart contract users.