used to express understanding, pleasure, pain, surprise, or the fact that you have noticed something

Ah, I see. 啊，我明白了。

Why has the train stopped? Ah, now we're off again. 火车怎么停了？啊，现在又开了。

Ah, Jessica, how wonderful to see you! 啊，杰茜卡，见到你真是太好了！

Ah, what a lovely baby! 啊，多可爱的宝宝啊！