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):
As I said, Nostro is also used for own accounts. The account refleting the position of the bank (its own money and assets, not the money or assets deposited by its clients) is also called a nostro, at least in Europe.