These little matted pieces are sometimes called slubs.

Slub yarns, as the name implies, possess bumps or slubs along the length of the yarn usually in uneven, unpredictable patterns.

A slub is an imperfection in yarn, seen as a thickened portion of the cord. In an uniforn length of yarn, the slub stands out from the rest of the yarn. This gives fabrics woven with slubs some roughness to their texture.

It depends on the type of irregularity. The most common is called a slub. Slubs are short sections of the yarn that are wider than the areas on either side and are either not twisted or not twisted as much as the rest of the yarn.

There are also snarls, knops and other irregularities that can be intentionally added for artistic effect.

When wool is drafted unevenly as it is spun it forms lumps in the yarn. The lumps, called "slubs," are thicker than the surrounding yarn and have little or no twist. This is generally considered a mistake which is common to beginning knitters but has become a popular intentional effect especially in art yarns.