pass away means die (e.g. His uncle passed away last night after a long illness.)
pass out means faint or to become unconscious for a short time (e.g. I was hit on the head and passed out.)I was hit on the head and passed out.
pass something out means give the same thing to people (e.g. I’ll pass out copies of the report. The professor passed the textbooks out before class.)
pass something up means decline (e.g. I have to pass the invitation up as I already had an appointment at that time. I passed up the job because I am afraid of change.)
pay back money, pay somebody back means return owed money (e.g. If you borrow money, you have to pay it back)
pay for something means be punished for doing something bad (e.g. I paid ten pounds for the meal. That bully will pay for being mean to my little brother.)
Pay off means completely repay a debt (e.g. The mortgage will be paid off in twenty years.)