Aspect ratio is merely a fancy way to describe the shape of the image. Is it a long, thin rectangle or more like a square?
Think of it as a measurement of the sides of the shape. So 16 x 9 is the same shape of a piece of paper that is 16 inches by 9 inches. Or it’s mathematical equivalent is 1.78 by 1. This is a nice balanced rectangle shape.
Standard definition TV is more like a square. It’s 4 x 3. But not really. It’s close to 4 x 3 so people call it that, but it is actually 1.33 by 1. It’s much more of a square than the 1.78 : 1 rectangle.
Many theatrical motion pictures are shot in 2.35 by 1. That is a really long rectangle called Panavision.
There are so many presentation formats and TV signals and so a lot of re-sizing has to be done to fit all these different rectangle shapes in their destination format.