A shadow on the ground.
Look at the shadow. The shadow itself is without substance, ephemeral, and yet it reveals the existence of something somewhere else.
The shadow on the ground symbolizes two realities - the ground, the here and now, the physical, the concrete; and the shadow, which reveals something beyond the here-and-now, beyond the physical.
Nothing is as insubstantial as a shadow, and yet the shadow reveals the silhouette of something that is beyond.
The essence of a succa is its shade. A succa that has more sun than shadow is invalid. Our Sages teach that when we sit in the succa, we are sitting in "the shadow of faith." The spiritual masters derived this phrase from a verse in the Song of Songs, "In His shadow, I delighted there and there I sat, and the fruit of His Torah was sweet to my palate." (2:4)
Faith is like a shadow. Faith is the knowledge of something that you cannot see. We can know there is a G-d but we cannot see Him. We can perceive the shadow of His existence, but we cannot see the Reality itself directly. We can experience closeness to G-d through tasting "the fruit of His Torah." We can experience the sweetness of that Existence that is beyond, but, for the very reason that He is beyond, we can never see that Existence. When Moshe asked Hashem to show him a revelation of that Existence, Hashem replied, "You cannot see My face, for man cannot see me and live." 
A succa is invalid if it has more sun than shadow because someone sitting in such a succa is not sitting in the shade of faith rather in the glare of the sun. In its essence, the sun does not allow for the existence of the shadow. Its unblinking eye leaves no place for the shadow. The nations of the world are compared to the sun. The sun says, "If you cannot see it, it doesnt exist. If it is not revealed, it isnt there." "Existence is bound by the revealed world," they say.
Hashem relates to his creatures measure for measure. When the nations of the world will come and complain to G-d that He did not give them the mitzvot that the Jewish People received He will give them an easy mitzva the sukka . To test their sincerity G-d will make the weather extremely hot. They will kick over the succa and leave. Even though the halacha says that someone who finds sitting in the succa uncomfortable is exempt, the nations of the world will not be content just to leave the succa, but will kick it over when they leave, thus showing their contempt for the mitzvot. To the nations of the world, the succa is no more than an uncomfortable booth. To the Jewish People it represents the resting of the Divine presence on this world.
The four walls of the succa symbolize this world of four directions. The roof of the succa is made from schach palm fronds, tree branches and the like. The word succa comes from the same root as schach. In other words, the roof of the succa is the essence of the succa. Just as the schach is placed over the succa, so the Divine presence hovers over this world. The gematria of the word "succa" is 91. If you take the gematria of the Tetragramaton, the ineffable four-letter Name G-d, and add to it the gematria of the way that name is pronounced, "Ad-onoy," you will find the sum comes to 91. In other words, just as G-ds Name hovers over the succa, it hovers over this world.
This world is like a succa, a flimsy affair not capable of protection, but the shadow of faith that hovers over someone who sits in the succa is stronger than a concrete roof a dozen feet deep.
The nation who dwells in the shadow of faith proclaims that existence extends beyond the here and now, beyond what can be perceived by the five senses of man. Faith is something that takes place in the shade. The nation that dwells in the shadow of faith draws that faith from the succa, for the shade of the succa is the shadow of faith.
But theres another side to the shadow.
Just as every movement, every flexing of a persons limbs and muscles is motivated by the living soul inside him, so too similarly all the powers of all the worlds, all their movements and their influences are motivated and affected by a vast chain reaction that starts with the actions of man in this lowliest of worlds and ascends to the highest places.
In the book of Tehillim it says, "Hashem is your shadow." When G-d created the world, He decided that everything that happens in creation would be governed by mans behavior. The spirituality that radiates from the highest levels of existence shadows our individual choices. Every kindness we do ascends through all the worlds to the highest places. There it triggers an influx of positive spiritual energy that descends again through all the worlds until it arrives back in this world. Every mitzvah rises to its highest spiritual source and causes a life energy that radiates throughout all the Creation. Hashem is our "shadow." He has committed Himself to run the world in synchronization with our actions.
As simple an action as sitting in the shade of the succa can cause the wind to blow and the rain to fall in its season, the sick to recover, famine to abate, and peace to descend on this troubled world.
- Shemot 33:20
- Talmud Bavli, Avoda Zara, 3a
- Nefesh HaChaim 1:7