An instrument that tells a financial institutions such as a bank to pay a certain amount of money, drawn on a particular account to another the person the check is issued to. The receiver of the check might have an account at another bank or credit union, to the institution is told where to send the money when the check is deposited.
Once the check is presented, the second institution notifies the one where the check is drawn of where to transfer the money. In this era, no money is actually sent since banking is ledger based. The settlement is handled at the reserve level which is nothing more than accounting.