Every dollar of money and money substitutes is held by someone. Money and money substitutes are valuable goods and so every dollar of them is owned by someone. All the funds in your checking account in any moment are being held by you and not spent. And the same is true for everyone else.
Money Stock = Money + Money Substitutes
To calculate the money stock, one adds up all the money, which is cash (currency and coins), and money substitutes, which are claims redeemable for money on demand at par (mainly checking accounts and saving accounts). The money stock that you are holding at any moment is the cash you have plus your checking and saving account balances. And the same is true for everyone else.