All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The scholars of Tafseer (interpretation of the meaning of the Quran) disagreed in opinion in regard to the interpretation of the letter or group of letters at the beginning of some Chapters in the Quran.
Some scholars considered them as part of the knowledge that Allaah kept secret for Himself only, so they did not try to interpret them.
Other scholars interpreted them but they differed in regard to thier meaning: some said that they are the names of some Chapters, as we say Chapter Hal Ataa Ala Al-Insaan (which is the first verse of chapter Al-Insaan [Quran 76]. We also say Chapter Alif Laam Meem Al-Baqarah [Quran 2], Alif Laam Meem As-Sajadah [Quran 32]…etc.
Others are of the opinion that these letters stand for some Names of Allaah. Others think that such letters stand for some different names from which they were taken. Al-Qurtubi reported from Ibn 'Abbaas who said that Alif stands for Allaah, Laam stands for Jibreel and Meem stands for Muhammad .
However, a different interpretation was reported from Ibn 'Abbaas himself.
To conclude, each scholar of Tafseer (interpretation of the meaning of the Quran) interpreted these letters in the way that he believed was sound according to his knowledge.
However, those who returned the interpretation of these letters to the Knowledge of Allaah have chosen the best interpretion.
In fact, this was reported from the four Caliphs and from Ibn Mas'ood who said that they are part of the Knowledge of Allaah which He reserved for Himself.
For more benefit, please refer to the interpretation of Ibn Katheer about the interpretation of the first letters Alif Laam Meem in Chapter 2.
Allaah Knows best.