When rendering live video streams to a fixed viewport, sometimes the video stream will have a different aspect ratio from the viewport. When it does, there are generally three ways to render it in the viewport: with cropping, black bars or stretching. Stretching will make the image appear taller or wider than expected. This is usually not wanted, so cropping or black bars are preferred.
With cropping, parts of the streamed image are removed to make the aspect ratio match the viewport. In this way, the entire viewport is filled, but part of the incoming video stream is lost. With black bars, the entire image from the streamed video is shown in the viewport, but part of the viewport is not used. The unused part can be filled with black or some other filler. Usually an application will have to choose one method or the other based on preference.