This is fine woolen yarn NM 40/2.
This tells the thickness/weight of the yarn, as well as how many threads are combined.
Classic fabrics is getting more and more different thicknesses and a good way to keep them apart is this NM.
Another good way to understand the weight is to compare them all with how many meters go in one gram of yarn. All this info you can find at the specifications.
All fine yarns are excellent for weaving, embroidery, sewing and whatever fine work you can do with it.
+/- 5g, 1g= +/- 20m