To get to this panel, press button Settings in JonDo's main window, then choose the entry Certificate Authorities from the list to the left.
A certificate is a (digital) container for a public encryption key, digitally signed by a certification authority (e.g. the JonDo team), that approves for third parties by the signature that this key belongs to a given person or organisation. If this approval for certificates didn't exist, anyone could pretend to be the JonDo team or a trusted Mix operator, and may operate honypot Mixes, for example, or may engage in the communication between users and really trusted Mixes/operators and read all data. Moreover, the insertion of faked update messages would be easy. (Further infromation concerning certificates...)