An internal Auditor is a person who audit a business or a person. They are often free-lance and which hired by outside company.
The auditor analyzes the functionality of a company, and search for problems with the business structure (if it exist).
An auditor looks into inter-relationship between co-workers, department, and sometimes Upper Management. They mostly deal with accounting works, to see if there are any loopholes in the system (etc. People stealing, short on inventory).
By getting an outside person to audit a company, it can eliminate the problem of bias.
P.S. you are getting an interview at Internal Auditing Department, and you don’t even know what they do? How did you search for the job at the first place?