Refers specifically to the outer covering of a tree trunk or branch.
この木の樹皮はとても厚い。
The bark of this tree is very thick.
樹
The craftsman carefully peeled the bark off the tree.
Do not peel off the bark.
The trunk of this tree is covered with thick bark.
Cork oaks are known for having thick bark.
木肌 refers to the surface texture or grain of wood/bark, whereas 樹皮 refers to the actual bark layer itself.
樹脂 (resin, the sticky sap of a tree) sounds very similar to 樹皮 (tree bark) but refers to a completely different substance.
A piece or sheet of 樹皮 that has been peeled off is counted with 枚, as it is thin and flat.
Compound of 樹 (tree) and 皮 (skin, bark).