Warming the occupant directly rather than warming the volume of air around them, so the room can be allowed to stay cool. Radiant and conductive sources put heat where a body actually is: under a table, against the feet, against the skin. Historically this was the normal way to be warm in winter, and it uses a small fraction of the energy that heating a whole house to a comfortable air temperature does. Distinct from zoned conditioning, which still heats air but in fewer rooms.
Ranked by how much of the heat produced reaches a person rather than the room. A source in contact with the body loses almost nothing on the way; one under a blanket loses a little; one radiating into open air loses most of it. Absolute output is not the axis here, because these are all small: a device delivering 50 W to someone's feet can replace a kilowatt of space heating, and that substitution is the point.