An image's Aspect Ratio, or AR, represents a comparison of its width to height. Notation for Aspect Ratio is normally in the form of X:Y, where X represents screen width and Y represents height. For example, a standard analog TV has an AR of 4:3 which means that for every 4 units of width it's 3 units high. This could be 4mm wide and 3mm high, 16in wide and 12in high, or 24m wide and 18m high. The exact dimensions aren't important, so long as the ratio between them is correct.
If your AR is