The speaker thinks about a river and a tree that shaped his life, yet he admits he knows almost nothing about them. He has never traced the river from its source to the sea, though his mother blessed him with its water after he was born. He never saw the tree grow up, though he played in its shade as a boy and gathered happiness there day after day.
Both become "fosterers": quiet carers who nourished him without his understanding where they came from. The speaker says this overwhelms him with sorrow, and the feeling seems to come from realising how much he has received without knowing its history. The last lines widen the thought. He has seen nothing of "the first morning" or the birth of the Earth, so the gap in his knowledge reaches back to the beginning of everything.
The mood is reflective and humble, with a touch of wistfulness. The broken lines and trailing dots make it feel like a mind pausing mid-thought.