Convalidation

"Convalidation is not simply a “blessing” of an existing union.
It requires that a new, free act of consent be made."
For Your Marriage

What is a convalidation?

"The Catholic Church believes that marriages entered into by non-Catholics in their particular church, or wherever they may take place, are valid. In the eyes of the Church, even two atheists or agnostics who are married civilly before a judge or magistrate enter into a valid marriage.

Convalidation Requirements: Catholics, however, are bound to observe a certain form of marriage ritual in order that their marriage be valid. Canon law—the law of the Church—requires that Catholics enter into marriage by free mutual consent that is witnessed in a church by an authorized bishop, priest, or deacon and at least two other witnesses. Marriages in which one or both parties are Catholic and which are not witnessed by an authorized bishop, priest, or deacon, or which do not receive proper permission to take place in another forum, are considered invalid in the eyes of the Church.

It may be that the Catholic who entered into marriage outside the Church did not realize that these requirements existed, but more often, it is because one or both of the spouses was not free to marry in the Catholic Church because of a previous marriage or because they were awaiting a Declaration of Nullity. Also, the Catholic partner(s) may not have been active in the Church and did not consider having a Catholic wedding.

The Church very much wants to assist these couples who later want to enter into valid Catholic marriage, and it offers them pastoral and spiritual support as they need it. When these couples are ready and free to do so, they celebrate what is called a convalidation, from the Latin word meaning “to firm up” or “to strengthen.” This is sometimes wrongly referred to as the blessing of a marriage.

It is important to realize that a convalidation is not merely a renewal of vows made previously but is a new act of consent by each spouse.
This new act of consent is essential to marriage, and the words that the couple expresses are the outward sign of the gift of self that they exchange.
This convalidation of marriage may be celebrated within Mass or outside of Mass, again depending on the particular situation of the couple.
If both are Catholic, it is fitting that the convalidation be celebrated within Mass. If one spouse is not, it is preferable that it be celebrated outside of Mass." Source: Together For Life

Why do we need to go through marriage prep for our convalidation?

A “convalidation” is not just the Church’s recognition of an existing situation, nor a blessing of an existing union. If both are baptized Christians, 
Convalidation = Sacrament of Matrimony with all the same “obligations and benefits,” or “goods and requirements”. 
A convalidation is not simply a renewal of the previous intention to marry, but the creation of a valid marriage in the sight of the Christian community.” USCCB 
Canon law requires proper preparation for entering the Sacrament of Matrimony.
Individual dioceses establish the guidelines for this preparation.
A civilly married couple needs to learn all that the Sacrament of Matrimony entails.

Make sure to read Convalidation: Bringing Your Marriage Into the Church (USCCB)