The aspect ratio of an image is the proportional relationship between an images width and height. Think wide-screen vs the squarer image on your TV.
A square has an aspect ratio of 1x1 that is to say that for every 1cm in width, it has 1cm height
Standard 35mm film has an aspect ratio of 3x2 and has a width of 36mm and height of 24mm
If you've used a photo processor such as Adobe Lightroom you will already be familiar:
Narrative has a list of common aspect ratios for you to pick: