It's the act of playing dumb. Like when your mom asks who ate the last piece of leche flan and you suddenly don't know anything. It's different from 'mangmang' (ignorant), which is a state. 'Magmangmang' is an action, a choice. It's the "I don't know what you're talking about" defense mechanism. In the age of fake news, some people choose to 'magmangmang' about facts.