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.

Additional Resources

Here is an example of uneven yarn: