What is the meaning of あ?

The hiragana syllable あ (a). Its equivalent in katakana is ア (a). It is the first syllable in the gojūon order; its position is あ行あ段 (a-gyō a-dan, “row a, section a”).

expresses surprise: Ah! Oh!

used to get someone's attention

used as a response to someone's call

first-person personal pronoun: I, me

distal demonstrative, something far off removed from both speaker and listener: that, yon

foot

ridge between rice fields; ridge between groves

to be, to exist

I; first-person pronoun

Source: wiktionary.org