Aspect ratio

What it's for

The aspect ratio is the shape of an image, independent of how big it is: 16:9 is the television and horizontal video, 9:16 the phone's vertical video, 1:1 the square, 4:3 the old monitor and most compact-camera photos, 3:2 the SLR. You need it before cropping a photo, before exporting a video and before ordering a print, because it's the thing that decides whether the subject stays in or gets cut off.

How it gets simplified

Width and height are divided by their greatest common divisor, that is reduced to lowest terms: 1920×1080 becomes 16:9, 3000×2000 becomes 3:2. It's the same arithmetic you do with fractions, and it always gives the shortest possible form.

Why a strange ratio sometimes comes out

Because not every size is a clean ratio, and the simplification says so honestly instead of rounding. 1366×768, the resolution of millions of laptops, is not 16:9: what comes out is 683:384, because that screen is 16:9 only loosely speaking. Ultrawides too: 2560×1080 gives 64:27 and 3440×1440 gives 43:18, while in the shop they're all called «21:9», which is a marketing name and not a fraction.

If the ratio you get is almost but not exactly what you expected, the problem isn't here: it's that the image doesn't really have that shape, and printing or uploading it will crop a few pixels. Better to know beforehand.

Nearby tools

To actually crop the image to the shape you need there is Crop image, and to change its size Resize photo. If the file is a video, what's really inside it (size, frame rate, rotation) is told by What video is this really. For print sizes there is Paper sizes.