The User ID of the certificate is fully valid.
The User ID of the certificate is marginally valid (e.g. it has been certified by only one marginally-trusted party).
The User ID of the certificate is never to be treated as valid.
The User ID of the certificate is ultimately valid (i.e., usually the certificate belongs to the local user themselves).
The User ID of the certificate is undefined.
The User ID of the certificate is of unknown validity.
The validity level of a certificate's User ID. Validity level tries to answer the question: "How strongly do we believe that this certificate belongs to the party it says it belongs to?"
Note that some OpenPGP certificates have multiple User IDs, and each User ID may have a different validity level (e.g. depending on which third parties have certified which User IDs, and which third parties the local user has chosen to trust).
Similarly, an X.509 certificate can have multiple SubjectAltNames, and each name may also have a different validity level (e.g. if the issuing CA is bound by name constraints).
Note that the GMime API currently only exposes the highest-validty User ID for any given certificate.