What this means is that most of the "money supply" is not in the form of cash.

There are three levels of money supply, right?  There is M1, M2 and M3 with M1 being the most liquid and M3 being the least.  Only about half of M1 is in cash in the US and none of M2 and M3 are in cash.

So most of the money in the United States only exists in the records of banks and other institutions.

But this is not new.  All that's new is that the money is now in computer files when before it used to be written down on ledger books or whatever banks used to use to keep their records.