Pater familias means "father of the family" in Latin. It is an old phrase that describes the man who is in charge of the family.
Imagine a house with a mom, a dad and their children. The pater familias is the dad in this case. He is the head of the household and makes all the important decisions for the family.
The pater familias was very important in ancient Rome. Back then, families were very big and the pater familias had a lot of responsibility. He not only had to take care of his own family, but also his extended family. His word was law and everyone in the family had to do what he says.
This meant that the pater familias had a lot of power. He could decide who would get married and who would inherit the family property. He was also in charge of the family's money and had to make sure that everyone was taken care of.
Today, the term pater familias is not used as much. Families are smaller and more equal. Both parents share the responsibilities and decision-making. However, in some traditional families, the father still holds the role of the pater familias.