Aircraft that fly and check the navigational aids for calibration/maintenance/certification purposes. Their presence reduces the airspace capacity, hence the delay you read about.
26. Flight Inspection/"Flight Check" Aircraft in Terminal Areas
26.1 "Flight Check" is a call sign used to alert pilots and air traffic controllers when an FAA aircraft is engaged in flight inspection/certification of NAVAIDs and flight procedures. Flight check aircraft fly preplanned high/low altitude flight patterns such as grids, orbits, DME arcs, and tracks, including low passes along the full length of the runway to verify NAVAID performance.
26.2 Pilots should be especially watchful and avoid the flight paths of any aircraft using the call sign "Flight Check." These flights will normally receive special handling from ATC. Pilot patience and cooperation in allowing uninterrupted recordings can significantly help expedite flight inspections, minimize costly, repetitive runs, and reduce the burden on the U.S. taxpayer.
— US AIP ENR 1. GENERAL RULES AND PROCEDURES
And from JO 7110.65, simply doing those procedures at say 3 AM when commercial traffic is light may not be possible:
9-1-3. FLIGHT CHECK AIRCRAFT [...] Certain flight inspection maneuvers require operations in close proximity to the surface. These maneuvers can only be performed during daylight visual meteorological conditions. [...]
Special thanks to @randomhead for the flight tracking:
By using Flightradar24's playback feature beginning at approximately 1400Z, a Super King Air 300 can be seen performing the flight check:
The airport was running in a north flow (departing and arriving using runway 01), and at approximately 1510Z the aircraft is seen flying what appears to be the RNAV (RNP) RWY 15 approach—the opposite direction to the rest of the traffic using the airport, necessitating delays.
The registration was blocked on Flightradar24, on the other hand ADS-B Exchange, which does not censor traffic, does not have a playback feature when the registration is not known.
Related posts and further reading: