Matrilineal refers to familial relationships that can be traced through a female. To follow the matrilineal line in your family, start with your mom.
In Latin, matri- refers to the mother, just as patri- refers to the father. Lineal is a word that refers to someone's lineage, or the line of people that came before that person; so the adjective matrilineal describes anything related to kinship through a female line. If the children in your culture take their mother's last name, and not their father's, this is a matrilineal tradition.