The Church in her wisdom, her history and the presence of
the Holy Spirit takes it upon herself to judge the validity of
marriages presented to her for judgment. If the presence of
some obstacle--called an impediment--is judged to have been
there at the time of a wedding ceremony, the Church issues
an annulment. She judges that while there may have been a
wedding ceremony there was no marriage in the eyes of God.
A wedding does not a marriage make. The parties are then
free to contract a valid marriage.