This is a method or sub-command of the Emoji command. It is not its own command.
This command can be used to create emojis or emotes on your server. It is most useful on mobile devices where the Discord User Interface can be a bit clunky and hard to navigate, although the command works perfectly fine on desktop and browser versions.
The emoji name can be anything you want, as long as it is alphanumeric (Contains only letters and numbers) and does not contain spaces.
The image you want to create as an emoji needs to be less than 256 kilobytes in size, which you can check by going into the properties of the file you're trying to upload.