Aspect Ratio

The video aspect ratio describes the shape of a video, and does so using width in proportion to the height.

16:9 is the standard aspect ratio for HD Televisions, monitors and mobile devices. This describes their shape in landscape orientation. 9:16 would describe the aspect ratio of video of the same shape but in portrait. 

This aspect ratio was devised as a way of effectively playing two common standards for TV and film 4:3 and 2.39:1 with the least amount of letterboxing. Letterboxing describes the areas of black on either the top and bottom or sides of the image when the shape does not match the display. 

Once this screen shape became the standard, 16:9 content was created to fill the whole screen.