The Sacrament of Matrimony signifies Christ's relationship to the Church and world, through the husband and wife. It signifies the never ending commitment that is made between humanity and God.
For a man and woman to be married, they must be free to marry, know what marriage is truly about, and want to marry.
The "Matter" is the Bride and Groom; the "Form" is the vows that they pronounce to one another. This is unique. It is the only sacrament that is not "Effected" by clergy. The Clergy is present to ensure the ritual is followed and as witness to the vows. The husband and wife truly marry each other.