A derogatory term for a slow, impatient movie goer, who only goes to films that require very little thought. A Popcorn Muncher wont like a film with too much depth or subtext, they need the plot spelled out for them, and hate it if you try to do something that's too different from the norm. They don't go to a movie for it's story, they just go as an excuse to shove their faces with over priced candies and popcorn while big, fast explosions happen in front of them.
Basically, Popcorn munchers are what Hollywood calls The Lowest Common Denominator they're the kinds of people who will ensure the latest Michael Bay explosion-fest is number one at the box office. They're the kinds of people who will tell you that the recent re-make of a classic film is better because it's more action packed than the original which is boring because it has "Too much talking".
Roger: "I can't believe that movie is doing so poorly at the box office, it was great!"
Brian: "I hear people don't like the twist at the end, and all that spiritual stuff"
Roger: "What? I thought that was so clever! I never saw it coming."
Brian: "Yeah, so does everyone else I've talked too. I have a feeling the people who hated it were just a bunch of Popcorn Munchers who went to it thinking it would be more action packed"