"Wewe hujambo?" is the short form of "Wewe hauna jambo?"
In this context 'jambo' means 'problem'
and Hauna jambo? is as follows:
You = Wewe u; have = na; problem (jambo)
Wewe u-na-jambo = You have problem
The negative is then made by fixing 'ha' before subject marker prefix 'u' to get:
Wewe hauna jambo can be shortened to Wewe hujambo?
Wewe hujambo = Don't you have problem?
Don't you have a problem, so how are you?
In English people sometimes say "what's up?" or even just the " 'sup?" as they walk by another person, but aren't really asking a question. It is technically a question, but really just meant to be an alternative to "hi."
It's the same here: "Wewe hujambo?" is shortened to "hujambo?" which is further shortened to "jambo" (mostly just by tourists). It is technically a question, but also used to say "hi."