They are two ATC positions whereby a controller, using a STARS display with an alert algorithm, monitors two parallel final approach courses to ensure aircraft do not penetrate airspace between those two courses. Depending on the spacing between the courses, procedures differ as to how a penetration of that airspace is handled, but basically the offending aircraft will be removed from the final approach course with the issuance of a radar vector. The monitor controllers don't have a discreet frequency for their position, but they have override capability on the tower controller's frequency in the event that they need to issue instructions to aircraft on those final approach courses. Otherwise, most pilots won't even know they are there.