The aspect ratio of an image describes the proportional relationship between its width and its height. It is commonly expressed as two numbers separated by a colon, as in 16:9 - or separated by an X, as in 16x9. If someone is speaking about a 16:9 aspect ratio, they'll likely say "16 by 9".
In the 16:9 image below, it is 16 units wide and 9 units tall. You're probably familiar with 16:9 on your TV or computer monitor.
Other common aspect ratios:
1:1 = classic Instagram
3:2 = many types of cameras, including 35mm
4:3 = older TVs and computer monitors
Note that a 16:9 screen is 25% wider than a 4:3 screen when they have the same height.