A furrow is a groove or a depression. Originally, furrows were created in soil for water to flow. Now, its most common meaning is of a deep line or wrinkle on the face, as in a "furrowed brow."
Furrow is related to the old English word farrow, meaning "to root like a swine," which makes sense, seeing as both acts create trenches in the earth. Sometimes a furrow simply means the impression left by a cart wheel or tire track in the mud. In a non-agricultural sense, a furrow is also simply a slight groove or depression or series of indentations in the surface of any object.