(おく)

おく
noun
interior, back
1. interior, back, depths
The innermost or furthest part of a space, away from the entrance.
It's at the back of the room.
The restroom is at the back of the store.
Please go a little further back.
There was a lake deep in the forest. It was a very quiet place.
When I searched the back of the drawer, old photos came out. Nostalgic memories came back to me.

(おく) indicates the deep interior or back of a space, opposite to the entrance (手前(てまえ)). It's used for buildings (部屋(へや)(おく) - back of the room), natural spaces ((やま)(おく) - deep in the mountains), and even abstract concepts ((こころ)(おく) - depths of one's heart). (おく)さん (okusan) is also a polite term for someone else's wife.