Elaborating on Kashida Justification

Kashida is basically a way to stretch the Arabic word by adding a dashed line be it short -, medium –, or long —. This option will allow the word to stretch sideways as I mentioned in order to close white spaces between words and eliminate so-called white rivers in a long paragraph.

As @Joel Cherney mentioned, Kashidas will not be visible unless proper justification settings are applied. Then, not all Arabic/Persian/Urdu/Kurdish fonts will respond to Khashida justifications fully as some of the Arabic fonts fall short of closing gaps properly.

I don't use the Justification panel, I use the Paragraph Panel rather to apply Kashida settings. Here in this video, you will see how different Khashida settings affect the text by stretching Arabic text sideway.

Note. I'm using an Arabic font named Winsoft Pro / Medium. This font used to come with old [ Adobe CS1 – Adobe CS6 ] Adobe applications.

Kashida can be added manually at certain times. Click on the dashed line ـ on your keyboard as shown on a Mac screenshot below, don't click on the Minus symbol -

There's not a lot of English-language documentation on using kashidas, but here's what there is! Note that they won't work at all unless the paragraph is full justified; if the paragraph is aligned any other way than full justification, they simply won't be inserted.