Unlike some other systems, Blender only renders what a camera sees. You can find out what the camera is seeing by pressing the 0 key on the Numpad.
When you pressed the 0 key on the Numpad, Blender displayed the Camera View. It also applied a gray mask called the passepartout to mark the limits of the image that will be created. The following screenshot shows the cube seen through the passepartout on the left and the rendering of that scene on the right.