No context is provided, but if my guess is correct, it would mean something like "on planet earth", or "with regard to planet earth", or "as far as planet earth is concerned". For example: اكبر حدث على مستوى العالم means: "The biggest event in the world". Where as: اكبر حدث على مستوى اليابان means: "The biggest event in Japan" Now, literally, على مستوى means "on the level (of ..)". You can say that descriptions like "biggest" etc are relative. The biggest object in my house is not the biggest object in the city, and the biggest event in some city is not the biggest event in the world. You can think of these as some sort of "levels", I think you will understand where this phrase comes from.

It is usually: علي كوكب الأرض علي سطح كوكب الأرض Maybe مستوي could mean also سطح in Arabic ? الشيء المستوي هو سطح If that is not the meaning, then as Hasen mentioned it could mean: "on the planet earth"

on the level of the Earth planet. & yea it could be majaz (something in the Arabic grammar) but it depends on the whole sentence & meaning, I can't tell.