In the context of
photography, this is the "light-tight box" photography teachers like to talk about.
Some camera bodies (like most 135 format) provide film transport and a shutter.
Others, like the Hasselblad simply provide a place to attach a lens (that can carry a shutter), a film magazine, and possibly a prism.
Declaring that "a camera body is just a light-tight box" usually has the objective of calling the attention on the lens, and possibly on the photographer's art: the camera is just a box, it is the human outside that does most of the work.