It's critical-parody about McD icon. The mascot, Ronald McDonald wear "Scream" mask and the slogan become "I'm leavin' it". The message: Cheap meat that produced with genetically-modified (animal) feed will cause a serious damage problem (health and environment), like a "Scream". If McD keep using cheap meat, consumers must say "I'm leavin' it".
Greenpeace Against Gen-Food