means A person who patrols and protects a forest, park, or wilderness area, or a member of a roving armed force trained to operate in rough country.

from From "range" — to roam over a wide area — which traces back through Old French "ranger" (to arrange in a line, set in order) and ultimately to a Frankish root for a row or rank. So a "ranger" was first simply one who ranged, who covered ground; by the late medieval period the word had settled onto the keepers who roamed and guarded English royal forests. The military sense — mobile fighters who range ahead of the main force — grew naturally from the same image of someone who travels far across hostile terrain.