Airspace is divided into ICAO classes, and Class G is uncontrolled airspace — the part where air traffic control (ATC) does not actively separate traffic. In practice this is most low-level airspace outside airport zones, which is exactly where drones usually fly.
"Uncontrolled" does not mean "no rules", though: drone geo-zones, the 120 m height ceiling and the duty to give way all still apply in Class G. And manned aircraft really do fly there — helicopters and gliders use Class G routinely.