At the end of the previous parsha, Vayelech, G-d told Moshe to write down a certain song to act as a witness to the people that G-d warned us about all the things, such as exile, that would befall us if and when we turned away from G-d.
This week's parsha, and the first aliyah, begins that song. The song begins with heaven and earth being called to witness that the Jews were properly warned. Rashi says that Moshe's thoughts were that he was human, and not going to live until the time the Jews would need to be reminded about this warning, so who could argue if the Jews someday claimed that they never accepted upon themselves the covenant with G-d. Moshe therefore calls upon heaven and earth to be witnesses because they exist forever.
In the second aliyah the song Ha-azinu continues with a description of how G-d originally chose the Jewish people and separated them from the nations. Then G-d took good care of us and bore us on eagle's wings protecting us, the metaphor being that an eagle carries its young above it, so that the mother shields her young with her own body from any arrows below.
Here, in the third aliyah, the song of Ha-azinu continues by telling us that G-d carried us on the highest places of the earth and gave us the best of the earth, but we abandoned the G-d that made us, and rebelled and pursued after alien gods.
In the fourth aliyah the song continues with G-d saying that He was offended with all we did, and His anger was kindled. Therefore G-d sent evil upon us, consisting of plagues and famine.
In the fifth aliyah G-d says that if we were wise, we would see the G-dliness in the ability of the Jewish to survive against impossible odds. When we fall, G-d asks us, where are all the other gods that we ran after, why aren't they helping us? We should see that G-d is the One and Only G-d.
In this, the sixth aliyah, G-d says that, in the end, He will avenge the blood of His servant's enemies.
In the seventh aliyah, after Moshe finishes teaching this song to the Jewish people, he said "pay close attention to all this, it isn't an empty teaching, it is our life, and with it we will long endure on our land." Later in the day that this song was stated, G-d tells Moshe to climb up the mountain Nevo, and see the land of Israel from afar. He is further told that when he is up the mountain, he will pass away, without entering the land, because of the incident at the waters of Maribah, where Moshe struck the rock, and therefore didn't sanctify G-d's name.
