Listen to two Egyptians talk for a minute and you'll hear ba'a four or five times. Ask what it means and they'll pause. It doesn't mean anything on its own. It tells you where the speaker is in the conversation.
The sound first
The word is written with a qaf, the letter that textbook Arabic says as a deep k. Cairo says it as a stop in the throat, the same catch as in the middle of uh-oh. So ba'a is two syllables with a catch between them. Our guide to the Egyptian q and g goes into why.
Moving things along
Tab ba'a is OK then. Tab on its own is well, or right, and ba'a adds the sense that a topic is closed and you're on to the next one. You'll hear this pair more than any other.
yalla ba'a is come on then, usually with a sigh. is already come on; ba'a adds the "we've been waiting".
Losing patience
'eeh ba'a? is so what now? is what, and with ba'a it's the what of someone who's been interrupted one time too many.
khalaaS ba'a is enough already. khalaaS is done, finished, which we cover in ma3lesh, khalaaS, tamaam. Add ba'a and it's the version a mother uses on the third request to stop.
Giving up
mish 3aaref ba'a is I just don't know. mish 3aaref is I don't know, said by a man; a woman says mish 3arfa ba'a. The ba'a is the shrug.
ya3ni ba'a is well, you know. It's what people say when they'd rather not say. ya3ni is I mean, and ba'a lets it trail off.
How to start using it
Don't. Not yet. From a learner ba'a sounds wrong until one day it doesn't, and the only way there is hearing it a few hundred times. Start with Tab ba'a.