Aspect ratio is the relationship between the width and height of an image or video dimensions. Aspect ratio is expressed by 2 numbers separated by a semi-colon.

The most popular aspect ratio would be 16:9 which means a width of 16 units with a height of 9 units (widescreen). 16:9 aspect ratio in video is the common 3840×2160 (4k), 1920×1080 and 1280×720.

An aspect ratio of 1:1 is a perfect square because the width has the same units as length. The need for social media video that gets viewed on a vertical screen means an aspect ratio of 1:3 would be ideal and fit the screen, whilst 16:9 would leave the mass of black space below and above the video.

Aspect ratio is more relational to the media holding its shape on different screen sizes which sets it apart from plain dimensions. You may have a 1920x1080p (16:9) monitor yet a 21:9 video has black bars on this screen. This is 21:9 media is ultra-widescreen and cannot fill a 16:9 screen fully.

The wiki gif above shows the differences with 4:3, 16:9 and 21:9 on those screen ratio.

21:9 numerically seems bigger than 4:3 but this is aspect ratios and the media cannot fill out a different aspect ratio without cropping.

Social media images such as Instagram and Twitter suit aspect ratio of either 1:1 for a square or 9:16 to fill the screen vertically.