The Meaning of the Arava:
The unity within the Arava: The Arava is a big bushy tree that has its branches and leaves grow very close to each other. This represents brotherhood and was thus chosen for the Mitzvah of unity.
The representation of the Arava within Jewry: The Midrash explains that the Arava represents the people who do not contain either Mitzvos or good deeds, as the Arava contains neither a good taste nor smell. On the Holiday of Sukkos, all Jews unify together, including such Jews. The Baal Shem tov explains that the Arava represents the simple Jew, who serves Hashem with utter simplicity.
 Siddur Im Dach Shaar Halulav p. 264
 Vayikra Raba 30:12