Something that's ribbed is covered in rows of raised lines, like a ribbed sweater or a pair of ribbed corduroys.
You probably have ribbed clothes in your wardrobe, like a ribbed jacket or ribbed knit cap. Your sofa might also be ribbed, with a pattern of raised bands on its surface. Other ribbed things include structures that are made sturdier using rib-like, parallel pieces of material. The adjective ribbed comes from rib and its Proto-Indo-European root meaning "to roof" or "to cover."