It's a respectful term used to address men, and can be used in a variety of contexts, similar to the English "Mr." or "Sir."
Here are some examples of how "bwana" might be used:
* Bwana Smith: Mr. Smith
* Asante, bwana: Thank you, sir.
* Bwana, naomba msaada: Sir, I need help.
It's important to note that while "bwana" is a respectful term, it can also be used in a more informal way, especially among close friends or family.