This is the 35th most frequent Japanese word.
あっ
"Ah" or "Oh" (an expression of surprise, realization, or recollection).
The word 'あっ' is used to express a realization or sudden recollection.
あっ、大事な書類を忘れちゃった。
Oh, I forgot the important documents.
Here, 'あっ' is used as an interjection to gain someone's attention before a request or correction.
あっ、そこに置かないでください。
Oh, please do not put that there.
In this context, 'あっ' is used to show concern or surprise when noticing something about someone else.