。
It was cool in the shade of the tree.
Literal
Tree-shade [topic-は] was-cool.
木陰 ('tree shade') is a compound noun meaning specifically the shade cast by a tree, distinct from 日陰 ('shade from the sun') generally. Note 涼しい — used here for a pleasant coolness, distinct from 寒い (cold, unpleasant) and 冷たい (physically cold to the touch). Japanese has finer-grained temperature vocabulary than English.