What does aspect ratio mean?

Monitor aspect ratio is closely related to monitor resolution, in that a monitor's aspect ratio is the ratio of its screen width to its screen height. For instance, a classic 1920x1080p display has an aspect ratio of 16:9 eg. for every 16 pixels in width, there are 9 pixels in height. 16:9 is the most common aspect ratio today, though there are certainly several widely supported options. "Ultra-wide" displays are usually 21:9, but there are also 32:10 and 32:9 displays ("Super Ultra-wide").