This is the third quest given to you by Louis Salvatore
Mr. Salvatore wants you to help him guard a transaction he wishes to do with the Enclave. When you talk to Mason for the details, you can either choose to go now, or wait. But don't wait too long or you'll miss the opportunity. When you accept to go with them you'll be sent to an outside location where 5 of your guards and two Enclave soldiers are making the deal (don't move around too much or they'll get mad and everyone will attack you).
If you don't screw up, the transaction will go off without a hitch, and you'll be able to return and talk to Mr. Salvatore. you get 1000xp for completing the mission, and if he asks you to become a made man of the Salvatores, and you accept you get an additional 1000xp. When you talk to Mason after the quest, he hands you an armload of items, containing a Laser Pistol.
Being a made man gives you respect from others, but also makes you unable to go into the other family hideouts (without a fight to the death, that is)