Concubinage is when a person has a romantic relationship with someone who they are not legally married to. It's like having a boyfriend or girlfriend, but without getting married.
In the past, some cultures allowed men to have more than one wife, but not all men could afford to support multiple wives. So, some men had another woman who they were not married to, but still lived with and had children with. This woman was called a concubine.
Today, having a concubine is not legal in most places. It is important for people to remember that it's important to treat everyone with respect and kindness, regardless of whether or not they are married.