The Arabic alphabet is consider an abjad, which means an alphabet that only contains consonants (though there are the weak consonants that function as long-vowels). In normal use, this leaves the reader to use their knowledge of Arabic word forms to infer the correct meaning.
In educational materials and religious texts though, it is important to know the exact vowel pronunciations. For this, Arabic uses a system called harakat. It is a set of diacritical marks that specify vowel sounds. Harakat are not used in general publications like newspapers, novels, instructions manuals, or streets signs.
Most harakat specify short-vowel sounds like /a/, /u/, and /i/, but there are two special cases that do not:
Shadda (شَدَّة) denotes gemination, the doubling of a consonant sound. Unlike other harakat, it does not represent a vowel sound. The word shadda is an example of itself with the doubled /d/ sound.
Sukuun (سُكُون) is another non-vowel harakat. It marks that the consonant it modifies is not followed by a vowel. Also when combined with a fatHah (َ ), it is used to represent the diphthong /ay/ with yaa (ﻱ) and /aw/ with waaw (ﻭ).