It's the Swahili equivalent of the English "You are" and is used to express presence or existence.
Here are some examples of how "upo" is used in a sentence:
* Upo wapi? (Where are you?)
* Mimi upo hapa. (I am here.)
* Upo sawa? (Are you okay?)
* Nimefurahi upo. (I'm glad you are here.)