A filbert is a narrow, flat brush with hairs that come to a rounded point. Used on its side, a filbert gives a thin line; used flat it produces a broad brush stroke; and by varying the pressure as you apply the brush to canvas, or flicking it across, you can get a tapering mark.
If the filbert has hog or bristle hairs, these will wear down with use. The photo shows (from left to right) a brand-new, never-used filbert, one that's done several miles of painting, and a very old one.
A filbert my favorite brush shape because it can produce such a variety of marks. Most of my paintings are done with a No.10 filbert. I don't throw away worn-down filberts as they can be useful for dry brushing; I don't feel sorry for them as I bash the hairs to spread them out.
See Also: Know Your Painting Brushes: Hairs and Bristles
Buy Direct: Filbert Brush