An alcohol-free event. Pop is the drink being served instead of alcohol. Kids who are straight-edge have these kind of parties, along with normal kids when their parents are around and they aren't allowed to drink.
"Man, do you think we should go to that party at Ross' house tonight?" - Person A
"No, that's gonna be a pop party, lets go to James' party instead"- Person B
A political party that "pops-up" one day claiming to be revolutionary. When it comes to actual elections, the party often fails miserably and disbands quickly after. The Brexit Party, Change UK, Alba Party and ATWA are all examples.