"Song of Myself" in total is a poem in which Whitman examines what it is to both be an individual and to be part of a larger world. Ultimately, he reveals that while everyone is an individual, no one is - we are all part of each other. Inextricably intertwined would be a good way to describe it - unavoidably and completely mixed in with everyone else.
In excerpt #46, Whitman covers two topics, which are themselves interconnected. He talks about travel and knowledge. Travel is a metaphor for experience, which brings knowledge. He says that he has the "best of time and space", because it is his bit of time and space and he continues on a "perpetual journey". This is the journey of life, and he is alone in it. It is his own because no two people can have the exact same experience. However, he leads others with him, and he goes where others have gone before. He can not travel the road for anyone else, as they must do so themselves. However, we (the human race) share the journey.
Whitman then describes reaching the crest of a hill, looking at the sky, and asking if he will every know all there is to know in the universe. The universe replies that he will not. Whitman speaks to "dear son" as a stand in for all of the human race. As the universe told him he will not know all the answers, he says the same to the "son". He tells "son" that he can sit awhile and renew himself, but eventually he must go forth on his own journey and experience life.