What is a Custodian?
A Custodian is an entity (such as a broker-dealer, bank, or transfer agent) that holds any securities on behalf of all investors (who are the "beneficial owners" of the securities).
This means you do not actually possess the shares, convertible notes, or SAFEs. Instead, the Custodian holds them on your behalf. The Custodian also votes these securities and signs any documents on their behalf, following the direction of the Lead Investor.
The finance lingo is that the Custodian holds these securities in "street name" on behalf of the "beneficial owners".