Parshat Haazinu begins with the words, Haazinu Hashamayim Ve'Adabeira Ve'Tishma Haaretz Imrei Pi. Listen O Heavens and I Will Speak and the Earth Will Hear the Sayings of My Mouth.
This sentence can be divided into two halves. The first of the heavens which is active. Moshe Rabbeinu is calling on the heavens to actively listen, 'Haazinu' as he speaks and to the earth however he speaks in a passive manner. The earth will hear, rather than actively listen, and rather than actively speaking, will hear the words of his mouth. Then the sayings fall like rains and dew to the earth.
What are the heavens and the earth then? One might say that the heavens which actively listen are those who study the word of G-d and participate actively in studying the Torah. Their teaching and transmitting it to the earth, to those of the ordinary people, the 'amei haaretz', 'the people of the earth' who do not actively seek it out and participate in its study but passively receive and accept it.
Then when it says 'Ve'atzar et Hashamayim' and the heavens will be shut, "veLo Yihye Matar', there will be no dew and as a consequence, 'Ve'Haaretz Lo Titen et Yevula', the earth will not longer be productive. Once those who actively learn Torah stop teaching it to the ordinary people, they cease to be spiritually productive and catastrophe comes to the Jewish people.
Alternatively, the 'heavens' are those lofty people who are too arrogant to listen when he speaks and accordingly must be commanded by Moshe Rabbeinu to listen and must be spoken to directly. While the 'earth' are the humble people who will naturally listen without having to commanded because the words do not have to penetrate their arrogance.