An aspect ratio is defined as a mathematical ratio, with two numbers separated by a colon. So that 4 inches wide by 3 inches high would be 4:3. Aspect ration will always be width:height. We use aspect ratio to indicate image sizes in order to make sure that the images you upload are displayed in the best way
The aspect ratio though is not simply just a measurement of width and height. It is reduced to the smallest usable ratio (using math concepts from elementary school). So for a rectangle 16 inches wide by 9 inches high, the aspect ratio is also 4:3 (a common aspect ratio in still photography).