What does `ls --directory` stand for?

$ man ls
...
-d, --directory
              list directories themselves, not their contents

The current directory is represented as . so that's what ls -d is listing.

The directories inside the current directory are contents of the directory, and are therefore not shown with this option.

I use the -d option in an alias to display hidden files and directories

alias l.='ls -dC .* --color'

Without -d, this will list out the contents of the hidden directories too, which isn't what I want.

Another use for it is when I want to see metadata of a directory using the -l option, not its contents:

$ ls -ld playground
drwxr-xr-x 72 zanna zanna 12288 Mar  1 23:10 playground

If you want a list of directories in the current directory you can use

ls -d */

Using ls only with -d is almost useless. It gives information about the directory itself. It does not list its contents.

That is why you do not see a list of directories. The directories that you expected to see are the "contents" of the current directory

If you run simply ls -d it shows the current directory by ..

If makes sense to run it with other keys like -l.

ls -ld will show permissions of the current directory.

pilot6@Pilot6:~$ ls -ld
drwxrwxr-x 1 pilot6 pilot6 2570 Mar  4 12:14 .

You can also see permissions of any other directory like

ls -ld /bin

pilot6@Pilot6:~$ ls -ld /bin
drwxr-xr-x 1 root root 2584 Feb 25 15:19 /bin

If you run ls -l without -d you will have the list of all permissions for files and folders in the current directory. If you do not need it, that's a good example for ls -ld usage.

You can list directories in the current one by

ls -d */

Using ls -d by itself is fairly useless because without a parameter it always returns .. After you specify a parameter it makes sense though. For example if your user name is rick and you want to see all the directories in your home use:

$ ls -d /home/rick/*/
/home/rick/AAC/        /home/rick/EnhanceIO/         /home/rick/silentcast/
/home/rick/assembly/   /home/rick/EnhanceIO-master/  /home/rick/Templates/
/home/rick/bin/        /home/rick/log/               /home/rick/test/
/home/rick/Desktop/    /home/rick/Music/             /home/rick/tmpe/
/home/rick/Documents/  /home/rick/Pictures/          /home/rick/Videos/
/home/rick/Downloads/  /home/rick/Public/