What he (probably) meant by it (who can say for sure now?) is that of all the possible means a government has to control and influence its people, the issueing of money is the most powerful one. Even more powerful than law.
By creating new money, the government can decrease the value of the existing money in circulation, thereby lowering the buying power of those holding it. In turn it can use the created money to buy assets on the free market. It's not fair competition.
This quote is often used in (political) statements against fiat money. They reason that by inflating the money supply, governments are actually stealing the wealth from their people without them (directly) noticing it. It is much easier to just create new money than to raise taxes. Tax revolts were quite common back in the old days, but inflation revolts? None would be the wiser until it was too late. "Read my lips, I will not raise taxes.". He says nothing about national debt, eh?
The United States, before they became independent of England, had issues with their (forced) use of the English pound, a currency which they did not control. The US constitution actually spends quite some words on trying to prevent US money from ever becoming fiat. Money should belong to the people was the founding fathers' opinion, and not to the head of state. Thomas Jefferson has even been quoted as saying that
I believe that banking institutions are more dangerous to our liberties than standing armies.
So there you have it. Money is more powerful and dangerous than both laws and standing armies.