some youtube videos are just a rectangle box

[extreme pcgamer]

some videos on youtube when i enter full screen mode i get the full picture. but other videos when i enter fullscreen the entire video is in a rectangle box. and the rest of the screen up and down is black why is this is this a monitor issue?

 

[James Mason]

Because it's a video that was shot in a 4:3 aspect ratio.
https://en.wikipedia.org/wiki/Aspect_ratio_(image)