“Ah” is a sound English speakers make when they react to interesting new information, or information that they have been looking for. 

Ah, okay, he says it’s ten dollars.

If you’re stuck in traffic, and your friend is looking at the traffic app on her phone, she might say:

Ah, there’s an accident two miles ahead. 

We also say “ah” to note mild surprise:

Ah, they have my favorite chocolate!