what in real terms causes one nations currency to be more valuable than another?
Exchange rates are prices.  Just like any other price, they change based on supply and demand.  Country A's currency appreciates relative to B if the citizens of country B want county A's currency.  Why does B want A's currency? Typically for the purpose of buying the goods, services, and investments produced by A.

So what would be some examples of events that would cause A's currency to appreciate relative to B?


If A starts producing some goods B wants, or producing them efficiently, then B will want A's currency in order to buy them
If A has desirable investments (savings accounts, financial assets, or real assets) and B wants a place to save for the future
If A expands their money supply more slowly than B, which reduces the available supply of A's currency relative to B's.
If A's currency is useful for some other purpose, like acting as a medium of exchange between B and another country C.