Mostly-uncontrolled airspace (flights do not need clearance from air traffic control), where ATC keeps aircraft away from each other where possible, but all pilots still need to "see and avoid" nearby traffic. Many countries do not use this airspace class.
Class F airspace is one of the seven airspace-classes; it is uncontrolled airspace, in that clearance by air-traffic-control is not required for any aircraft, but ATC is still responsible for separation between instrument-flight-rules flights as much as is possible. However, since the information available to ATC about flights in class F airspace is generally at least somewhat limited, all flights still need to be careful to "see and avoid" any potentially-conflicting traffic (ATC provides separation advisories to all flights as far as is possible, in order to warn pilots if there might be another flight passing close by, but their ability to do so is often limited by lack of information about some or all flights in the area).
Many countries, including the United States, do not use class F airspace, instead jumping straight from class-e-airspace to class-g-airspace.