This is based on a classic saying, "you scream, I scream, we all scream for ice cream". In case you're not a native speaker or it's not obvious from text, the saying is based on the fact that "I scream" and "ice cream" sound similar, so it's a bit of a pun. However, without the last part of the saying which references "ice cream", it just sounds like there's a bunch of screaming, which is pretty terrifying!