A financial custodian is a company that has physical possession of your financial assets. It is a financial institution that holds your money and investments for convenience and security.
The custodian is often referred to as the gatekeeper of assets whose function is to track monies and assets moving into and out of the account, and they are entrusted to render regular financial valuation of such assets held in custody through the Custodian Agreement (link).
A custodian financial institution holds customers' securities for safekeeping to prevent them from being stolen or lost. They may hold stocks or other assets in electronic or physical form.