When the proportions of an image don’t exactly match the proportions of the screen where the image is displayed, you might see black bars in the empty space of the screen. Depending on where the black bars are placed, this is referred to as Letterboxing, Pillarboxing, or in some cases what is informally known as “Windowboxing”.
Black bars can be removed in 2 ways:
1) Stretch the video to fill the entire screen (suitable for files that have small black bars or part of the video can be cropped, so the final movie looks good).
2) Inserting pixelated video under the original which is great if you have recorded vertical movie using your phone.