My understanding that are the internal agency that investigates leaks and illegal activity of agents within the CIA. Just like an internal affairs department from a police departments.
This is very serious and since the person comes from this office, this is very bad for the democrats. This agent was trusted to investigate others and their wrong doings and this person was doing the same. This is going to go very deep.
Inspector General (OIG)
The Office of Inspector General (OIG) promotes efficiency, effectiveness, and accountability in the administration of Agency activities. OIG also seeks to prevent and detect fraud, waste, abuse, and mismanagement. The Inspector General is nominated by the President and confirmed by the Senate. The Inspector General, whose activities are independent of those of any other component in the Agency, reports directly to the D/CIA. OIG conducts inspections, investigations, and audits at Headquarters and in the field, and oversees the Agency-wide grievance-handling system. The OIG provides a semiannual report to the D/CIA which the Director is required to submit by law to the Intelligence Committees of Congress within 30 days.
Google on CIA / IG office and you will see where they have a hand in many different types of functions, providing something like a quality assurance review on previous investigative determinations.