What on earth

"What on earth". I would like to know what does it mean?

It just means -- tell me what is going on. What is happening? Explain to me. What in the world is going on?

"On earth" is an idiomatic way of intensifying the words "what", "where", "why", etc. for emphasis. For example, "What makes you think that?" is a simple request for information. But "What on earth makes you think that?" implies "What reason could there possibly be in the whole world for you to think that?" Related idioms are "whatever" and "what in the wide world?" (VSD)