A more thorough explanation:

Concubinatus is a term from Roman law that refers to a long-term relationship between a man and a woman who are not legally married. This type of relationship was not against the law, but it did not have the same benefits as a legal marriage. It was often a relationship between a man of higher social status and a woman of lower social status. For example, a wealthy man might have a concubine who was a slave or a woman from a lower social class. Concubinatus was regulated by law, but it was not considered to be the same as a legal marriage.