The law requires an independent person to sign off that a firm’s financial statements are “true and fair” and have been prepared using the relevant legislation. In the UK, that’s the Companies Act.
The firms that carry out the work needed to give that opinion are auditors. These are typically firms of qualified chartered accountants who spend time at the firm interviewing managers and checking records.
They are looking for instances of fraud, or simply large errors in either the numbers that make up the accounts, or the narrative that appears in, say, the accompanying director’s report.
Note that the auditors do not usually prepare the accounts – that is the job of the firm’s finance director. Also, ultimately, they are doing their work on behalf of a firm’s shareholders.