The speaker seems to be calling out to a boy named Billy, who is alone on a dark hill in a storm. The tone is part scolding, part tender: "silly boy" sounds like what you say to someone you love and are frightened for. The weather is oddly personified. Thunder keeps him company, rain quenches his thirst, and lightning lights his way down to the valley. These are small, strange comforts, and they make the storm feel both kind and threatening at once.
The valley's "limpid monotony" of spilling streams sounds calm, almost dull, which contrasts with the wild hillside. Then the poem turns. The speaker cries "Oh for my Billy boy!" and ends with a plain, urgent instruction: don't go under the apple tree. The poem never says why, and that silence is what unsettles. Judging by the nursery-rhyme rhythm and the speaker's pleading, the mood is anxious and haunting, a lullaby with fear underneath it.