An annulment (or decree of nullity), is a declaration by the Church that, at the time a couple attempted to exchange wedding vows, an essential element was lacking in the consent of at least one of them. As a consequence, the union which followed such consent was not adequate. The absence of adequate consent means that, in the view of the Church, there was not a true marriage.
Where a marriage is declared not to have been binding, there is no obstacle to either party remarrying in the Catholic Church.
An annulment is not a divorce. It is a declaration that, after careful investigation, in the eyes of the Church, a valid marriage did not exist.
For more information about annulments, contact the Marriage Tribunal of the Archdiocese of Melbourne.