"Aha" is something we say when we suddenly find something, or understand a complicated idea, after a lot of difficulty and time.

Aha! I found it. I had been spelling it wrong.

Aha! You have a new boyfriend, don’t you!

Or imagine that your friend told you last week that she has some good news for you. When you meet her, she tells you that she’s having a baby. You say:

Aha! I knew it!