In this aliyah, Hashem tells Moshe Rabbeinu that his days are drawing near to die, and commands him to call Yehoshua so they can present themselves together in the Ohel Moed. There, Hashem appears in a pillar of cloud to give Yehoshua his charge and to prepare Moshe for his departure from this world.
Hashem forewarns Moshe that after he lies with his fathers, the Bnei Yisrael will go astray after foreign gods, forsake Hashem, and break the covenant. In response, Hashem's anger will flare up, and He will hide His countenance, leading to many evils and troubles that will cause the people to realize that Hashem is no longer in their midst. To serve as a perpetual witness for these future times, Hashem commands Moshe to write down the shirah of Ha'azinu and teach it to the Bnei Yisrael.
The Torah is telling us a very deep thing here. When Klal Yisrael is going through a hard time, they look around and say, "Surely it is because our God is not in our midst that these evils have befallen us." Lichoira, this sounds like a beautiful teshuvah. They realize they are missing Hashem's presence! But the very next pasuk says, "Yet I will keep My countenance hidden on that day." How can this be? If they realize Hashem is missing, why does the hester panim continue?
The Chassidishe sefarim explain a tremendous yesod here. The mistake of the people was that they said, "Hashem is not in our midst." They thought that because they fell spiritually and were experiencing tzaros, Hashem had completely left them, chas v'shalom. They believed that in their low state, they were entirely disconnected. But that itself is the ultimate hester panim! The moment a Yid thinks, "I am too far gone, Hashem is not with me in this dark place," he is missing the basic emunah that Hashem is always there, even in the hiddenness.
This is our avodah when things get tough, either spiritually or physically. The yetzer hara wants you to believe that because you stumbled, or because you are facing a heavy nisayon, Hashem has abandoned you. The teretz to this is to know with absolute pshat that even in the deepest darkness, "Anochi haster astir", it is still "Anochi," Hashem Himself, who is hiding there. He is right there with you, waiting for you to call out to Him.
Today, if you feel a sense of distance from Hashem, or if you are facing a struggle and feel spiritually cold, do not say "Hashem is not with me." Instead, stop and say out loud: "Hashem, I know You are right here in this room, in this struggle, hiding behind this cloud, and I am holding onto You."