Bar means son in Hebrew. Mitzvah means commandment. Noun pairs such as this usually have an implied of between the nouns, so bar mitzvah means literally a son of a commandment. So much for the literal. Culturally, the term is used to identify the point in the life of a son when that son takes on adult responsibility for his own actions. (The corresponding term for a girl is a bat mitzvah). This is considered a big thing. Pre bar mitzvah, the parents are responsible. Post bar mitzvah, the son is responsible. With responsibility comes the ability to lead communal prayers and be counted toward the quorum of 10 required to hold a communal worship service, so it's traditional to celebrate.