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 investors do not actually possess the shares, convertible notes, or SAFEs. Instead, the Custodian holds them on their behalf. All voting rights of securities are delegated to the Lead Investor, who directs the Custodian on how it should vote the investor securities. This makes the Custodian the only entity listed on the cap table, because it is the record holder of all the securities.
On Wefunder, XX Investments LLC - an SEC-registered transfer agent - acts as Custodian for all securities sold on the platform that use the Custodian structure.