A demand account is that account in which money can be used at any time, that is, it is an account in which money maintains its main characteristic, liquidity. So, an example is a checking account, where deposits are made, but at any time you can withdraw your money.
There are other types of accounts where money can not be used quickly, that is, loses its immediate liquidity. These are the investment accounts where you make an investment and can only use the money after a certain amount of time.