The aspect ratio of an image is the ratio of its width to its height. The larger the ratio, the ‘wider’ the image; the smaller the ratio, the ‘squarer’ the image.
This is the aspect ratio of full HD and 4K UHD video and it’s been widely adopted as the aspect ratio for domestic TVs and computer monitors. The 16:9 ratio means that the picture is 16 units wide by 9 units high. These units can be anything from pixels to centimetres to inches, but the point is that the ratio between them always remains the same at 16 wide to 9 high.