Very simply, Priority Pass is a membership program that allows you access to ‘Business Class’ airport lounges. You can access these airport lounges EVEN IF YOU FLY ECONOMY.
It doesn’t matter which airline you’re flying with, or which class of cabin your ticket is in, IF YOU HAVE PRIORITY PASS MEMBERSHIP, you can enter the lounge.
So you’re flying with a budget airline like Ryan Air, or Air Asia, and your ticket cost $20? That’s fine.