A CTR, or control zone, is controlled airspace around an aerodrome that extends from the ground surface upwards. Its purpose is to protect aircraft arriving at and departing from the aerodrome, so all traffic inside the zone operates under air traffic control.
Drone flights inside a CTR — the Riga CTR, for example — fall under geographical zone restrictions: a flight typically has to be coordinated in advance, and in the immediate vicinity of the airport drone flights are excluded entirely. Always check the zone conditions on the map before flying.