"Bar Mitzvah" literally means "son of the commandment." "Bar" is "son" in Aramaic, which used to be the vernacular of the Jewish people. "Mitzvah" is "commandment" in both Hebrew and Aramaic. "Bat" is daughter in Hebrew and Aramaic. (The Ashkenazic pronunciation is "bas"). Technically, the term refers to the child who is coming of age, and it is strictly correct to refer to someone as "becoming a bar (or bat) mitzvah." However, the term is more commonly used to refer to the coming of age ceremony itself, and you are more likely to hear that someone is "having a bar mitzvah" or "invited to a bar mitzvah."
So what does it mean to become a bar mitzvah? Under Jewish Law, children are not obligated to observe the commandments, although they are encouraged to do so as much as possible to learn the obligations they will have as adults. A Jewish boy automatically becomes a bar mitzvah upon reaching the age of 13 years, and a girl upon reaching the age of 12 years. The Bar Mitzvah ceremony formally, publicly marks the assumption of that obligation, along with the corresponding right to take part in leading religious services, to count in a minyan (the minimum number of people needed to perform certain parts of religious services), to form binding contracts, to testify before religious courts and to marry.