In the third aliyah G-d commands the manner in which the Jewish tribes are to camp around the Mishkan, each with a flag of a unique color which Rashi says was the same color as that tribe's stone on the breastplate (Choshen Mishpat) of the Kohane Gadol. Rashi also says that the way the tribes camped in the desert is the way Yakov Avinu's sons carried their father Yakov out of Egypt to his burial in the cave of Machpella, and as we know, the tribes descended from Yakov's sons.
This aliyah also gives the order of the departing of each tribe whenever the Jewish people journeyed in the desert.
As we've learned previously in the Torah, the Jewish people would know to journey when the cloud of G-d's glory moved. Until this time the cloud rested on the Mishkan.
