If you have Iranian friends, I'm sure you've heard them use bah bah at some point. Iranians use this when they are pleased with something. If you cook, they might say "bah bah, it smells wonderful!"
If you are invited to someone's house, they may say "bah bah! Khosh āmadid!, welcome!" when they open the door. They are happy to see you. Or if they haven't seen or talked to you in a while, they may say bah bah! Che ajab!, What a [pleasant] surprise!
Mothers may also use this word to mean food when they are talking to infants. For example, bah bah bokhor, eat your bah bah.
And like many other words, you can use it in a sarcastic sense. "You went out and got drunk the night before your exam?? Bah bah! [Cheshmam roshan!]"
(And of course, the opposite of this is ah ah, which expresses disgust.)