From Halopedia, the Halo wiki
The Auditor was an official in the Forerunner ecumene. In approximately 97,745 BCE, the Auditor received a report from the Prelate detailing what would later prove to be the first Flood invasion of the Milky Way since the human-Forerunner wars 9,000 years earlier.
"Auditor" is defined as a person who in charge of evaluating or assessing something. This reflects his task, seemingly geared toward monitoring suspicious phenomena across the ecumene and further reporting them to the appropriate authorities.
List of appearances