Since Matrimony is a sacrament given by Christ to his Church, the Church has the authority to determine if a marriage is valid or not based on a set of criteria: the spouses were free to marry, they freely exchanged their consent, they had the intention to marry for life, to be faithful to one another and be open to children; and their consent was given in the presence of two witnesses and before a properly authorized Church minister. Without these criteria being met, a marriage is not valid. So an annulment is when the Church has decided this marriage was never valid to begin with.