Everyone in the office is using this word these days. Jia(1) you(2) literally means "add oil".
When people are overwhelmed by the amount of work they have to do, they'll complain about it together. And when the ranting session is over, they try to cheer themselves up by saying jia you! jia you! There's nothing else they can do except press on.
It's another way of saying "keep going!" or "keep up the fight!"