What does Uneven Yarn mean?
A type of yarn that is known for being uneven, or slub, with thicker areas along the length of thread.
Heddels explains Uneven Yarn
Uneven, or slub, yarn is a yarn with thicker areas along the length of thread. Though it was originally considered a flaw, many manufacturers now purposely use uneven yarn to create fabrics with more character and texture. Uneven yarn is sometimes used for denim to create a material that will develop more unique fade marks over time. Using uneven yarn for both the warp and weft threads produces crosshatch denim.
Here is an example of uneven yarn: