Aspect ratio is the proportional relationship between an image's width and height. A player can preserve that shape with borders, crop part of the picture or stretch it to fill a display. The appropriate presentation depends on the source and viewer's preference, but stretching changes the apparent shape of objects and people.
A practical example
A wide film shown on a television may have bars above and below the picture when its full composition is preserved. Filling the entire screen can instead require cropping the sides or changing the original proportions of the image.
A common misunderstanding
Black bars do not necessarily mean that video is missing or playing at low quality. They can be the correct result of displaying the source's original shape on a different-shaped screen.
Related terms
Stable technical definition. Editorial approach