When you sally forth, you rapidly set off on an adventure or outing. Your mom might think it's funny to yell, "Sally forth!" when it's time to catch the bus for school.
The phrase sally forth is a somewhat old-fashioned way to say "embark" or "get going." It comes from the primarily military noun sally, which has been used since the 16th century to mean "a sudden springing forth of besieged troops to defend against attackers." These days, most people use this phrase in a joking way: "Let's put on our costumes and sally forth for trick-or-treating!"