In general, a message authentication code confirms that a particular message comes from an official source. If you’re in charge of a nuclear bomb silo and you get a message to nuke Paris, the MAC makes sure the directive comes from military high command and not some hacker who hates the Mona Lisa.
In the financial world, these MACs are used for login purposes. They help keep accounts (like bank accounts or brokerage accounts) safe by confirming that, yes, you were the one who wants to wire $5,000 to...well, we can't tell you or we'd have to shoot you (with an iPhone camera).