Aspect ratio refers to the ratio of the width and height dimensions of a video. You can calculate the ratio by dividing the width and height by the greatest common divisor of both.
For a video that is 1920 pixels wide and 1080 pixels in height, the greatest common divisor is 120, resulting in an aspect ratio of 16:9.
The two most common aspect ratios for video are 4:3 (e.g. 640x480) and 16:9 (1920x1080 or 1080p). With vertical video, popular for mobile device recording, the common aspect ratio numbers get reversed and 16:9 becomes 9:16.
The square aspect ratio, 1:1, is also becoming popular because of social sites like Instagram that allow square video as an option.