Sukoon   ـۡ

Sukoon is a sign similar to a little ( ۡ ) that we put above the letter to indicate the absence of a vowel. It simply means do not make the sound of a vowel on that letter.

For example: When you say Islam, (the /s/ sound has no vowel), in Arabic rules, the /s/ gets a Sukoon, and it attaches to the letter /l/.