If you add 'on earth' to any direct or indirect question, it adds emphasis. It reinforces the idea that the speaker is confused or mystified, or can't imagine what the answer might be. It's quite common, both in colloquial speech and informal written language. For example, if you have no idea at all what something means, you might say: 'What on earth does this mean?'. This phrase has the advantage of showing your confusion and irritation, while avoiding actually swearing. A more vulgar version would be 'What the xxx does that mean?', where 'xxx' is a vulgar word of your choice. This phrase is often used as a kind of word-play for titles, games and slogans. For example, I'd guess that the example you gave - with a capital 'E' for 'earth' - is the name of a geography quiz of some kind.

It's just a strong way of saying "What is it?"

It is used all of the time when trying to describe an unusual thing or sonmething new or different. It is a perfectly valid expression.