A directorate is defined in RFC 2418in the following way:
"In many areas, the Area Directors have formed an advisory group or directorate. These comprise experienced members of the IETF and the technical community represented by the area. The specific name and the details of the role for each group differ from area to area, but the primary intent is that these groups assist the Area Director(s), e.g., with the review of specifications produced in the area."
Directorates that cover whole IETF Areas, such as the Security Directorate which covers the IETF Security Area (SEC), typically assist the Area Directors with reviews. Some directorates are focused on a specific technology or industry activity, such as the Internet of Things Directorate. These span across areas and do not typically review specifications produced in any particular area. Lists of both types may be found on the IETF Datatracker: