Rain tapping on a window reminds the speaker of a boyhood friend who, oddly, never existed. The imagined friend would throw gravel at the glass to call him outside. Even without that signal, the speaker would sneak out, climb down the walnut tree, and go looking, because someone might be out there.
The strange, lovely part is that something always was. Footsteps race across the lawn, something rustles in the compost pit. The poem never says what it is, and that keeps it open: an animal, a neighbor, or the boy's own wishful imagination.
The speaker's feeling seems to be a mix of loneliness and thrilled expectation. He wants company badly enough to invent it, yet the night answers back. The mood is secretive and charged, and the ending, with the boy under the blankets listening to his pulse, captures fear and excitement tangled together. That drumming blood is the proof that he made contact with something.