No need to be sorry, but yes, Wikipedia articles can be a bit "vague" sometimes, it was just the first reference I found, maybe this one on investopedia is better, though it is also seemingly putting an accent on foreign currency/foreign banks, which I don't believe is the only use (I mean the concept applies to banks in a same country as well, AFAICR):
https://www.investopedia.com/ask/answers/051815/what-differe...