The rule of thirds is the phenomena that humans are drawn to the intersecting points of the rule of thirds grid when looking at an image. Because of this, the grid can be used as a tool to help create composing visually appealing images, whether it's the moving images of filmmaking or stills photography.
Rule of Thirds in Cameras
Almost all cameras nowadays have what is called a grid overlay, which overlays the rule of thirds lines to the image in your electronic viewfinder in real time. This makes it much easier to frame subjects correctly in the frame and gives support in the creative process of staging the scene and composing your shot.