Peace be upon him. These words can be used as Duaa. It means you are asking Allah for mercy, and the angles send you Duaa asking Allah to forgive you, and as you send prayers to the prophet you get peace , forgiveness and Blessings from Allah and Ameen from his angles. ( this is also said at the end of every prayer, before Salam) May Allah send his blessings and peace upon the prophet Mohammed;peace be upon him).