tree bark
Refers to the outer covering of a tree trunk or branch.
この木の皮はとても厚い。
The bark of this tree is very thick.
彼は木の皮を剥いで何かを作っている。
He is peeling off the tree bark to make something.
Compound of 木 (き, 'tree') + の (possessive particle) + 皮 (かわ, 'skin, bark'). A straightforward descriptive phrase.