What is the meaning of أ?

The first letter of the Arabic alphabet, called أَلِف (ʔalif) and written in the Arabic script; followed by ب (b).

introduces a positive yes-no question

introduces a negative yes-no question

indicates an exclusive disjunction with أَمْ (ʔam, “or”), as in alternative questions

indicates an inclusive disjunction with أَمْ (ʔam, “or”) that denotes sameness, indistinguishability, unimportance, insignificance, or indifference

vocative particle

A letter of the Kashmiri alphabet, written in the Arabic script; preceded by آ (ā) and followed by ٲ, a vowel.

A letter of the Khowar alphabet, written in the Arabic script; preceded by ﺁ and followed by ب, the second variant letter of alif.

The first letter of the Kyrgyz alphabet, written in the Arabic script.

The second letter of the Ushojo alphabet, written in the Arabic script; preceded by آ and followed by ا.

The first letter of the Malay alphabet, written in the Arabic script.

The first letter of the Persian alphabet, written in the Arabic script.

Source: wiktionary.org