In the first 4 lines of section 14 of "Song of Myself," Whitman describes looking up at the sky and seeing geese fly overhead. When a goose cries out, Whitman feels a connection to it, like the goose is calling down to him "an invitation" to join them. This description is representative of Whitman's entire body of work in several ways, mostly related to the themes that he expresses.
The first is that Whitman has a pervasive theme of feeling connected and at one with the earth, people and nature around him. He expresses this sentiment in most of his poetry; so, even though, as he mentions in this section, "the pert may suppose it meaningless," Whitman sees the geese overhead as just one more indicator that they are all connected and one in this life here on earth. The second theme that is seen in these lines is Whitman's focus on nature; all throughout his poems he describes nature and its beauty, going about its natural processes. Whether it is geese overhead, the circle of life, or taking a walk on a hunt, Whitman celebrates and rejoices in the beauty of nature and its natural cycles. Another theme that is expressed in these lines that is also common to his poetry is the purpose of life; each person, each bird, each tree, all have their own purposes and roles in life. He says he is able to find the geese's "purpose and place up there" in the sky--to him, they serve their purpose, just as he does. Each of us has an invaluable and unique contribution to life, and Whitman celebrates that in his poetry.
This passage is also written in very simple, natural free-verse poetry that sounds almost like prose; that is representative of Whitman's style throughout much of his poetry too. So, there is that similarity also. I hope that those thoughts helped; good luck!