Muqatta‘at, which in Arabic means abbreviated or shortened letters, are symbolic letter combinations. They appear respectively at the beginning 29 chapters of the Quran in the form of Alif Lam Mim, Ha Mim, Nun, Ayn Sin, Qaf, etc. They also appear as chapter names, such as Ta Ha, Ya Sin, Sad and Qaf.

First let us learn the Translation from Ahlus'Sunnah wa Jamaat: 

[al-Baqarah, 2:1]

الٓـمّٓ ۚ

Alif, Lam, Mim. (Only Allah and the Messenger [blessings and peace be upon him] know the real meaning.)

✅ It is important to learn, these Huroof E Muqatta‘at (shortened letters are a part of the Qur'aan)

✅ Qur'aan is taught to the Prophet ﷺ by Allāh Subhanahoo wa'ta Aala directly. The Prophet (titled as) al-Ummi (who imparts to the people from Allah the news of the unseen and knowledge and secrets of socio-economic disciplines of life without himself being taught by any human in the world);

✅ These are the secrets 

According to these translators, Allah (Alone) knows it's meaning however we Ahle Sunnat wa Jamaat believes Allah and the Messenger blessings and peace be upon him know the real meaning.

These below translators have translated as

❌ they have no meaning in themselves,

❌ it was not, in fact, revealed by Allāh to anyone and is known only to Him.

❌ Allâh (Alone) knows their meanings and I.e. NOT the Prophet Muhammad ﷺ (if it's about the original meaning then yes but here denying the knowledge of The Prophet Muhammad ﷺ)

Now let us analyse the Translations other than 

Ahlus'Sunnah wa Jamaat:

Alif Lām Mīm – letters like these begin certain sūrahs of the Qur’ān. Although they have no meaning in themselves, they do however have a purpose. Since there is divine wisdom contained in everything in the Qur’ān, these letters point to the miracle of the Qur’ān, made up of the letters which make up the words we speak. Therefore, when sūrahs begin with letters in this way, they are usually followed with a verse about the Qur’ān, as we see in this sūrah.

— Abridged Explanation of the Quran

Alif, Lām, Meem.[1]

— Saheeh International

[1]These are among the fourteen opening letters which occur in various combinations at the beginning of twenty-nine sūrahs in the Qur’ān. Although there has been much speculation as to their meaning, it was not, in fact, revealed by Allāh to anyone and is known only to Him.

Alif, Lam, Mim. (These are the names of three letters from the Arabic alphabet, probably indicating that this inimitable Revelation, the Qur'an, is made of this Arabic alphabet. Only Allah Knows their meaning here).

— Dr. Ghali

Alif, Lam, Mim.

— Tafheem-ul-Quran - Abul Ala Maududi

[1]The names of letters of the Arabic alphabet, called huruf muqatta'at, occur at the beginning of several surahs of the Qur'an. At the time of the Quranic revelation the use of such letters was a well-known literary device, used by both poets and orators, and we find several instances in the pre-Islamic Arabic literature that has come down to us. Since the muqattaat were commonly used the Arabs of that period generally knew what they meant and so they did not present a puzzle. We do not notice, therefore, any contemporaries of the Prophet (peace be on him) raising objections against the Quran on the ground that the letters at the beginning of some of its surahs were absurd. For the same reason no Tradition has come down to us of any Companion asking the Prophet about the significance of the muqattaat. Later on this literary device gradually fell into disuse and hence it became difficult for commentators to determine their precise meanings. 

It is obvious, however, that deriving right guidance from the Quran does not depend on grasping the meaning of these vocables, and that anyone who fails to understand them may still live a righteous life and attain salvation. The ordinary reader, therefore, need not delve too deeply into this matter.

A.L.M (letters that Allah (God) only knows their meaning).

— Dr. T. B. Irving

 Alif-Lâm-Mîm. [These letters are one of the miracles of the Qur’ân and none but Allâh (Alone) knows their meanings.]

— Muhammad Taqi-ud-Din al-Hilali' Muhammad Muhsin Khan