It is the basically about the relationship between the width and height of the picture and more particularly, how that is displayed on your TV. Movies have almost always been filmed in an oblong shape but TV's have, until recently, been basically square. This has therefore meant a compromise by either cropping the picture or modifying the picture to fit onto a square screen, the result of which means that the picture doesn't appear the way it was intended. The aspect ratio you choose will determine whether you see the picture in full resolution and in the same dimensions as it was filmed, or you see a distorted or cropped image. Widescreen TVs attempt to remedy this problem, so now we have a choice, the aspect ratio numbers have become more significant.
Well, just to make your purchase decision even more difficult,
aspect ratio is also used in the specifications of bar codes, to
describe the relationship of the width of the wide and narrow bars. Remember that when you are in the checkout line to pay for your aerodynamic boob tube.