When you sound the short blasts a second time, the camps that lie on the south side are to set out. The blasts are to signal them to set out.
KJV
When ye blow an alarm the second time, then the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys.
ASV
And when ye blow an alarm the second time, the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys.