Sukun (jazm)    Link sign

Sukun (jazm) resembles a semi-circle which appears over a consonant.  its function is to link its previous letter and form a single sound.   A letter with sukun sign must link  to its any previous letter ( to its right) with its own short vowel and form a single sound.   Alternatively if a consonant with its own short vowel and its next letter has a sukun then both letter must join and form a single sound. 

Read about silent mute letters
Read about shaddah (Tashdid) 

No Arabic word may begin with sukun