The easy answer is that it makes global investors pour their money into countries so they can get a piece of the return. As interest rates go up, interest in that country's currency goes up. If a country raises interest rates over an extended period of time, this can cause a broad trend against other currencies. Money just continues to pile into these currencies until there is any indication that the party might end soon.
The downside of this approach to trading is that it's very risk sensitive. Anything that could affect economies globally can shake an interest rate trade to the core. This type of shakeup doesn't come often, but when it does, it leaves disaster in its wake for anyone that isn't prepared.
There are always multiple factors that move a currency, but interest is one of the number one factors, only followed by risk.
Linkback: https://www.forex.zone/forex-education/29/how-do-interest-rates-affect-currencies/798/
