What is aspect ratio ?
Aspect ratio is the ratio of the width of an image to the height of the image (x:y).
What are black bars ?
Black bars appear when the aspect ratio of the content playing isn't matched with the aspect ratio of screen. Depending on the type of mismatch between the picture and the display's ratio, the black bars appear in different places. Content wider than the screen it is played on will have horizontal black bars, while content that is taller will use vertical bars.
Examples :
