In order to use Wealthfront’s Stock Investing Account, you must also have a Wealthfront Cash Account. The funds in your Cash Account will be used to purchase securities in your Stock Investing Account, and when you sell securities, the proceeds will be swept to your Cash Account.
It is standard for brokerage firms to give clients a place to store their cash that they are preparing to invest. At Wealthfront, the Cash Account serves this purpose. Your Cash Account comes with a high APY and FDIC insurance through partner banks. More details can be found here.