In section 6, what is the major symbol used and what does it represents?
In section 6, Whitman explores first through the eyes of a child what grass is. They both venture mild conjecture that the grass has something to do with life, but the section quickly shifts to death. Whitman considers how the grass grows untended out of the earth, a place bodies have been buried. He imagines that a blade of grass could represent any number of types of people: young men, old grandmothers, or even babies who were ill at birth and really never had a chance in the earth. But the spring of a blade represents something to Whitman:
The smallest sprout shows there is really no death,
And if ever there was it led forward life, and does not wait at the end to arrest it,
And ceas'd the moment life appear'd.
All goes onward and outward, nothing collapses,
And to die is different from what any one supposed, and luckier.
I think he is saying these lines that when a new blade sprouts out of a grave, that dead person actually just gave new life. Death begets more life, and that is something to think about thematically, not just as a symbol. You can connect that to things that "die" in your life or circumstances figuratively.