As we know, many Surahs of the Noble Quran begin with Arabic alphabets.
For example, Surah Al-Baqrah and Al-Imraan (and I think more) begin with Arabic alphabets,
"Alif. Lam. Mim." ألمSurah Al-Araf begins with
"Alif. Lam. Mim. Saad." المصSurahs Ta Ha and Ya Sin begin with alphabets
"Ta Ha" and
"Ya Sin" طه and
يس respectively, which have been chosen as the titles of these two Surahs.
Similarly Surah Qaf begins with alphabet
"Qaf" or
ق and Surah Saad begins with alphabet
"Saad" or
صAll these alphabets are always included in the text and recited as part of it.
Opinions differ to their significance. There are two views in this regard, as follows:
1) The prevalent view is that they indicate some mystic words. This is apparently the opinion of the ulemas but they haven't mentioned what mystic words these alphabets might refer to.
2) Others have opined that these alphabets are merely the initials of the scribes.
In my personal view the second opinion seems more correct.
Concerning the first opinion, if these alphabets referred to some mystic words, Allah Almighty would surely have told us the meaning and significance of them, just as all important matters mentioned in the Quran about Faith and practical life have been clarified and identified. But since the Quran does not clarify nor discuss the meanings of any of these Arabic alphabets, therefore it's far more likely that these alphabets are the initials of the scribes who noted the Divine Revelations as the Prophet recited them.
And only Allah knows best.
Discussed in much greater detail in the thread:
Significance of Huroof-Muqataat (حروفمقطعات)