In this recipe, you added the GAME_ROOM sprite to the scene, then added a GameObject containing a Sprite Mask component as a child of the GAME_ROOM GameObject.
You set the sprite of SpriteMask to GAME_ROOM_windowMask. This was a special image that only contained the rectangular area of the night skyscape:
The black rectangle in the GAME_ROOM_windowMask image defines the only places where a masked sprite will be displayed – that is, the rectangle where the window appears in the GAME_ROOM image.
By adding the blueBird GameObject to the scene as a child of GAME_ROOM and setting its relationship to Sprite Mask, only the parts of the blueBird image that appear inside the rectangular mask image of GAME_ROOM_windowMask will be seen by the user.
You can think of a sprite mask as a piece of cardboard with a hole cut out of it – we can only see images...