What is a "sanatio in radice"?

Did you know that if a Catholic is married to someone who refuses to enter a Catholic Church, and will have nothing to do with the Church, the Catholic party can have their civil marriage validated?  This canonical process is called a "sanatio in radice," meaning a healing of the invalid nature of the marriage so that the marriage becomes a recognized marriage according to the law of the Catholic Church, even without a ceremony.  This assumes that there is no existing impediment like a previous marriage that would need an annulment.