中 (naka) means “inside” | MotiTan Japanese Dictionary

More examples

箱の中にりんごがあります。bako no naka ni ri n go ga a ri ma su

There is an apple inside the box.

子どもたちは部屋の中で遊んでいます。ko do mo ta chi wa heya no naka de aso n de i ma su

The children are playing inside the room.

中 (naka) refers to being surrounded by or included within a larger space, set, or group.

内uchi: The part of a place, object, or group that lies within its boundary.
内側uchigawa: The area on the inside of a boundary or object.

ある範囲や集まりを基準にして、内側にあることやその一部であること。a ru han i ya atsu ma ri o ki jun ni shi te uchi gawa ni a ru ko to ya so no ichi bu de a ru ko to

中 (naka) is an inherited Japanese word, but its older word formation is not certain; the character 中 (chū, "middle") was later used to write it.

In phrases about groups, 中 (naka) can mean being among the members rather than being physically inside a container.