On Sukkot, we bring together four different species to fulfill the most fundamental mitzvah of Sukkot, which is in Leviticus (23:40).
The commandment is to take the fruit of a beautiful tree (etrog), the branches of a date palm (lulav), myrtle tree (hadas) and willow tree (aravah), bring them together and shake them in all directions at various points during the holiday, rejoicing for seven days.
There are many theories as to why we use these species. One is that each represents a different type of Jew. The etrog, enjoyable to eat and smell, symbolizes one with Torah knowledge and good deeds. The date palm’s fruit, delicious but without aroma, is like a person with Torah knowledge but no good deeds. The myrtle, without fruit but with a nice scent, is equivalent to the Jew with good deeds but no knowledge. The willow branch is he who has neither Torah knowledge nor good deeds.
Holding the four together then symbolizes the unity necessary for the Jewish people. For it is said by our Sages that the collective prayers of the Jewish people are stronger than any individual’s, and that as a unit, our good deeds and knowledge are so much more than on our own, that we become more deserving of God’s blessings.