The impalpable sustenance of me from all things, at all hours of the day,
The speaker is like a hungry man – he's constantly hungry for the "things" of the world. By things he really he means "everything" – all the hard matter that makes up the world, including people.
Fortunately, these things manage to feed him, to give him "sustenance […] at all hours of the day." He doesn't know how they fill his craving: it's mysterious or "impalpable." They just do, kind of like milk and cookies in the middle of the night.
The simple, compact, well-joined scheme—myself disintegrated, every one disintegrated, yet part of the scheme, The similitudes of the past, and those of the future,
The speaker describes the world as you might describe a well-made car, as a "simple, compact well-joined scheme." Things just fit together.
The speaker accepts his own "disintegration" (ahem, death), as a natural part of this larger unity. From the perspective of the scheme, things don't change between past and future.
The glories strung like beads on my smallest sights and hearings—on the walk in the street, and the passage over the river,
Everything he sees and hears gives him pleasure, and he compares the objects of his perception to beads on a necklace. Talk about a guy who appreciates the small things in life.
The current rushing so swiftly, and swimming with me far away, The others that are to follow me, the ties between me and them,
Metaphor alert! It's important to know that the current of the incoming tide that causes the water to rush past him also represents the distances of time and space that separate the speaker from other people. He doesn't warn us of this sudden use of symbolism, he just – pardon the metaphor – dives right in.
In his mind, he imagines "swimming" with the current and being carried "far away." The idea that a person can figuratively be in two places at once is central to this poem.
The certainty of others—the life, love, sight, hearing of others.
So we're still talking about the "glories" of the surrounding world.
Among these glories are the future passengers whom the speaker addressed in Section 2, as well as the idea that people everywhere are connected by certain basic emotions and capacities, like love.