What's "on earth" mean? In what context can I use this?

Native speaker’s answer
Rebecca
"On earth" is used in a question form to create emphasis. It's like it exaggerates what's being asked by using the size of the earth and everything in it in relation to something or someone. So you can use it when you want to create dramatic emphasis in a question form. Ex: What on earth is a cat doing in my apartment? I'm allergic to cats! Ex: Who on earth is that? I LOVE their outfit. Ex: Why on earth would I believe you?