Twenty-nine chapters of the Quran begin with short sequences of Arabic letters. We’re not actually certain what these “mysterious letters” mean.
The 20th sura (chapter) of the Quran is about Moses – the burning bush, the pharaoh of Egypt, the magicians’ snakes, the exodus, the Red Sea, the golden calf – and about Adam and the angels. It begins with two Arabic letters: طه (Ṭā Hā); it is from those letters that the sura gets its name. Twenty-eight other suras begin with similar short sequences of letters: ʾAlif Lām Rā in the 10th sura, for example, or Ḥā Mīm at the start of the 40th sura. Together, these opening marks are known as the Muqatta’at, the mysterious letters. And we don’t know exactly what they are.
This being the domain of religious scholarship, theories abound about the mysterious letters. Proposed solutions include the following:
We do not know, and we cannot know; understanding of these letters resides – appropriately – with Allah.
The letters represent or connect to the names of Allah.
The letters are initials of the scribes and readers of each surah, or the original owners of the manuscripts used to produce the Quran.
The letters are a means of organising and arranging the surahs in a specific order.
The letters are the names of each surah and relate thematically to the subject of that surah – the Ṭā in Ṭā Hā, for example, connects to “serpents” and therefore the snakes of Moses and the magicians.
The letters carry hidden mystical meanings, either numerological or symbolic – Sufi orders have picked up on this possibility and have put forward a few theories.
Some of these solutions are more plausible than others, of course. The third link below is a nice summary of the various proposals and the evidence (or lack of evidence) for each of them.