X509 certificates are used for “secure” internet connections. They are issued by certificate authorities. To become a trusted certificate authority a root certificate has to be installed. Operating system and web browser vendors deploy a lot of those certificates – usually only from commercial companies (since non commercial organizations cannot be trustworthy).
CACert is an organization which offers X509 certificates for free. Unfortunately there root certificates are not preinstalled on Apple’s devices. But this can be easily done by the users.