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.