Wikipedia
Muqattaʿat
The Muqattaʿāt ( ''ḥurūf muqaṭṭaʿāt '' "disjoined letters" or "disconnected letters"; also "mysterious letters") are combinations of between one and five Arabic letters figuring at the beginning of 29 out of the 114 surahs (chapters) of the Quran just after the basmala. The letters are also known as fawātih or "openers" as they form the opening verse of their respective suras .
Four surahs are named for their muqatta'at, Ṭāʾ-Hāʾ, Yāʾ-Sīn , Ṣād and '' Qāf''.
The original significance of the letters is unknown. Tafsir ( exegesis) has interpreted them as abbreviations for either names or qualities of God or for the names or content of the respective surahs.