If something is ribbed, it has raised lines on it, like a row of little ridges. The picture behind the word is a rib in the body: a long, raised shape with a dip beside it. You see it on cloth (a ribbed jumper, ribbed cuffs that hug the wrist), on corduroy, on metal sheets, on some leaves and shells, on tyres, and on some tools with a grip. Knitters and clothes shops in particular use it as a normal, not-fancy word. People also use it for things designed to grip or to stretch and spring back. Though actually, the lines do not have to look like bones; any regular raised stripes can count, and the word does not by itself tell you the material — only the surface.