When you have fur string (aka string made of fur) what do you think of? Hopefully wool yarn.
The readings for these kanji are the kun'yomi readings. With 毛 you should have learned this reading via the vocab word in the previous level. With 糸 it's actually the main reading for the kanji, so you should already know it.
Please give me some pink wool.
Grandma gave me a wool blanket she made herself.
These jeans are actually made of wool.