[aniro]

"The money supply measures reflect the different degrees of liquidity—or spendability—that different types of money have. The narrowest measure, M1, is restricted to the most liquid forms of money; it consists of currency in the hands of the public; travelers checks; demand deposits, and other deposits against which checks can be written. M2 includes M1, plus savings accounts, time deposits of under $100,000, and balances in retail money market mutual funds."

From the Fed itself:

https://www.newyorkfed.org/aboutthefed/fedpoint/fed49.html