Five short lines describe a pebble, and the poem gives it a "he," as if it were a person. That small choice invites us to read the pebble as a stand-in for someone, or for a way of living: plain, steady, and unbothered by the world around him.
The pebble isn't "marked or moved" by "glitter or tint," so flashy colors and shiny distractions slide right off him. Meanwhile "the dull being / that ever flows past him" suggests a constant stream of ordinary existence, like water running over a stone. He doesn't fight it. He "absorbs" it.
The speaker's feeling is hard to pin down, and that's part of the interest. The tone could be admiring, since the pebble seems calm and unshaken. It could also be quietly sad, since being unmoved can look like being cut off from life. Judging by the word "dull," the mood is still, muted, and a little lonely, but peaceful too.