One of two things:
A bank in which you can deposit gold, that gives you a certificate redeemable in gold, but which does not keep 100% of the deposited gold in a vault. In effect, you are buying the note. To be realistic, today you may simply deposit the gold, and have it electronically available to you via a debit card, et cetera…but it is only keeping a percentage uncommitted to other uses.
A bank in which you deposit money, for example dollars today, that does not keep all of your dollars reserved for you, but keeps immediate access to a given percentage of all dollars deposited, much as with the gold notes above.
Obviously it’s actually one thing, and whether it’s gold or notes is irrelevant, but the above addresses the two real-world applications we’re likely to discuss, thereby keeping anyone from thinking their version doesn’t count.