under a tree
Poetic or literary term for the area beneath a tree. In everyday speech, 木の下 (きのした) is more common.
We rested under a tree.
Standard kanji spelling.
Compound of 木 (こ, 'tree') + の (possessive particle) + 下 (した, 'under'). The reading こ for 木 is an older or poetic form.