large enough such there is a ready supply of capital in economy for the free efficient optimal exchange of economic goods and services
restricted enough such that the currency is not devalued as what happens when the supply of money exceeds the current demand, this fuels Inflation - Wikipedia which is bad
large enough such that the currency is not appreciating as what happens when the supply of money cannot satisfy the current demand, this fuels Deflation - Wikipedia which is bad
There are many so called 'definitions' of inflation/deflation nowadays. Your quote gives a true statement but does not tell the whole story.
The velocity (flow) of money is always perceived as the way it can influence price levels. If we think of money as merely a ledger (which is all it is) , showing the balances of debtors and creditors, then price levels will be influenced both by transactions, and by the confidence levels engendered by stock of debt or credit. The confidence (or fear in the case of debt) gives rise to changes in quoted price levels, but the actual transactions can be different (compare with say, the stock market system).