Crop a video to 1.91:1, the ratio behind Facebook and LinkedIn link cards and in-feed landscape video. A 1920x1080 source exports at 1920x1006.
Where the number comes from
The reference asset is 1200x628, which works out at 1.9108:1. Nobody chose 1.91 as a ratio; it is what that pixel size happens to be, and the ratio was named after the fact.
It is close to 16:9 at 1.778:1 but not the same. A 16:9 clip dropped into a 1.91:1 slot is usually cropped automatically off the top and bottom, which is fine until something sits near an edge.
The lightest crop on the list
Cropping 1920x1080 to 1.91:1 keeps the full width and takes 74 px of height, 37 from each edge. Almost nothing is at risk, but check any lower-third graphic or caption burned into the bottom of the frame, because 37 px is enough to clip the descenders off a small typeface.
Even dimensions
1920 divided by 1.91 is 1005.2, and H.264 will not encode an odd height. The export rounds to 1920x1006, so the delivered ratio is 1.9085:1 rather than 1.9108:1. The difference is under a pixel of visible framing.