Jaha can mean 'really?', 'right then', 'so', 'oh', 'ah, right!' and many more things, depending on the context.
One common use is to show understanding of something, for example if someone says Anna jobbar på Ikea (Anna works at Ikea), you could say jaha ('OK' or 'I see'). Or if someone explains something you've been puzzling over, you might say, Jaha, så det var därför (Aha, so that's why).
Or if you're told on your first day at work vi fikar tre gånger om dagen på kontoret (we have fika three times a day in the office) you could reply jaha ('I see' or 'oh, right').
You can indicate surprise, excitement, scepticism or any other emotion you want through the tone of your jaha. To emphasize surprise, an alternative response would be Jaså! which means something like 'Is that so?!'
You can also use jaha as a filler word to indicate a change of subject, or conclusion of something. For example, if your group of friends has finished eating at a restaurant and paid the bill, and you're sitting waiting for someone to make the first move to leave, you can say 'Jaha?' to indicate that you're ready. In this case, it's similar to 'Right then' or 'Shall we?' in English.
Or after a film has finished, or a conversation has come to an end (or even if you just want to indicate that you'd like it to come to an end!), you can say something like Jaha, ska vi äta nu? (Right, shall we eat now?', or a simple Jaha on its own.
Either way, it's a useful word to add to your Swedish vocabulary for plenty of occasions when you're not quite sure what else to say.
Jaha, nu förstår jag
Ah/Aha, now I understand
Jaha, ska vi gå ut nu?
Right then, shall we go out now?