This is the 5330th most frequent Japanese word.
in
The Japanese word "in" (院) refers to an institution, establishment, or building, such as a hospital, temple, or academy.
The phrase '部屋の中' expresses the 'interior of the room,' where the condition is described.
部屋の中は寒いです。
It is cold inside the room.
The word 'に' is used to indicate the locational aspect ('inside the box') where the apple exists.
箱の中にリンゴがあります。
There is an apple inside the box.
Here, 'に' paired with '中' specifies the location ('in the bookshelf').