Where 1920 × 1080 comes from
It is the pixel count of a 1080p frame, and it has outlived the screens that made it standard. It is asked for by video editors that want every clip on one timeline to match, by presentation software that scales anything else badly, by television-attached players that letterbox whatever does not fit, and by a long tail of upload forms that copied the number from something else. All of those want the same thing: 1920 columns and 1080 rows, exactly.
This page opens with those two numbers in the fields and the chain on. Drop a picture and the result is there before you touch anything. Whether the height stays at 1080 depends on the shape of what you dropped, which is the next section.
When your photo is not 16:9
Most cameras do not shoot 16:9. A phone photo is usually 4:3, a full-frame camera is usually 3:2, and neither of those fits 1920 × 1080 without something giving way. The chain is on by default and it protects the shape, so a 3:2 photo asked for 1920 wide comes out 1920 × 1280 — taller than you asked, and correct.
If the thing you are feeding really does need 1920 × 1080 and nothing else, unlock the chain and choose cover. You get the box exactly and a strip of the picture is discarded to make room. The nine-point grid decides which strip: centre by default, top-centre if there is a face near the top of the frame. Stretch is also there, and it will make everyone in the picture 33% wider than they are, which is almost never what anyone wants but is occasionally what a specification demands.
Questions we get asked
My source is smaller than 1920 wide. What happens?
The don't-enlarge box is ticked, so the output is capped at the width your source actually has and the page tells you the number it capped to. You can untick it and force 1920, and the result will be a 1920-wide file with no more detail in it than the original had. This page does not invent detail and does not pretend to.
Is 1999 × 1125 close enough to 16:9?
It is 1.7769:1 against 1.7778:1, which is inside a twentieth of a percent, so nothing will complain. The page prints the true ratio whenever the drift passes half a percent, so if you do not see that line, the drift is smaller than anything downstream will notice.
Why does the file still look soft after resizing down to 1920?
Almost always because the original was soft and the extra pixels were hiding it. Discarding three quarters of a photograph's pixels does not add focus. What resampling can do badly is add aliasing, and the stepped downscale here is chosen specifically to avoid that; the mechanics are set out here.