>>3396380This anon is correct.
Screen sizes differ and aren't usually the same aspect ratio of the sensor or film on which it was recorded. The safe area box lets you know what part of the recorded media will have action in frame and what part will be cut out.
The graphic here shows the safe areas on a frame of 35mm film for different applications. The red box is for Super 35, like for display on a cinema screen; the dashed box is for 4:3, the ratio of old TVs.
