The speaker starts out on an ordinary walk, noticing the squirrel, the child, the insects, the ants. Everything is familiar, and he feels "well and tall." When he stops and steps back for a wider view, that confidence grows into a sense of being important, as if the world he knows is his to understand.
Then the poem turns. Looking up at the stars, he can't grasp their distance or size, and he suddenly feels small. Picturing the view from a plane, where people are just "little dots," he extends that thought to himself: a speck on the earth, a speck in a single life. The repeated "unable to comprehend" and "unable to understand" make the ending feel humbled and a little dizzy. The mood moves from easy self-assurance to quiet awe, with a hint of unease about how much lies beyond what he knows.