A more thorough explanation:

A handfast marriage is a type of marriage that was historically characterized by the couple joining hands to conclude a marriage contract. It was often lacking only solemnization by clergy. In some cases, it was a betrothal with all the binding effects of a marriage, including conjugal rights and cohabitation, followed by a later formal marriage. In modern times, it can also refer to a trial or probationary marriage wherein the couple agrees to cohabit and behave as spouses for a definite period, usually one year, at the end of which they will mutually decide to separate or go through a permanently binding ceremony.

For example, a couple may decide to enter into a handfast marriage to test their compatibility before committing to a permanent marriage. They may agree to live together and behave as spouses for a year, after which they will decide whether to make the marriage permanent or separate. This type of marriage is not legally recognized in most jurisdictions, and its legal status is unsettled.