Don't feel stupid, Nolu . . . it's a relatively obsolete legal term used to designate jurisdictions that are ruled by what is called the "common law." States that call themselves Commonwealths still utilize common law. In this scenario, common law decrees that after a couple has cohabited for a set number of years, living as husband and wife, they are considered to be legally bound as if they had entered into a formal marriage.
Most of the time, when people use the term "common law" they don't really know what it means, Nolu.