Aspect ratio is measured between the height and the width. Aspect ratio is important because if your video is a different aspect ratio to what the are area on the monitor is a few things can happen. If your aspect ratio is bigger then it won’t be able to fit on the screen so it makes it smaller which will squash the image and you will be left with black bars at the top, you can even be missing some of the video. If your aspect ratio is smaller the same process will happen and it will try to correct it and it will make the video large which stretches the video, you can also be left with black bars on the sides.
This is what happen to the video when you film at the the wrong ratio so it has to render the video.
Image from: https://i.ytimg.com/vi/FOdi7Yo9SNc/hqdefault.jpg
“Most televisions and computer monitors currently available have an aspect ratio of 16:9, which offers a perfect fit for high definition television shows. However, movies are usually filmed with a ratio of 21:9, which will result in black bars at the top and bottom of the picture when it is displayed on the average TV. To fix this, some manufacturers are producing televisions with a 21:9 aspect ratio. The table above shows the most common aspect ratios.”
The image below shows what happens when you put a different video with a different aspect ratio to what the TV is, which shows your left with them black bars.