The aspect ratio in the "video realm" refers to the the height and width proportions of an image on a screen or monitor. The first number of the ratio is always the width of the image and the second number being the height. The ratio will define the shape of the image that is portrayed on the screen.
If an image ratio is too big to fit a screen or does not match a monitors screen, pillar boxing (the black bars you see on the top, bottom, or sides of your screens) is used.
Learn what kind of aspect ratio you need for your project.