The aspect ratio describes the frame size of your video as a ratio of its width to its height, as measured in pixels.

For example, a video shot in NTSC DV format has a frame size of 720 by 480 pixels, giving it an approximate 1.33:1 aspect ratio:

720 (frame width) ÷ 480 (frame height) = 1.5

1.5 x 0.9091 (pixel aspect ratio) = 1.36365.