My brother says there’s a ghost in the shed
Who hides under the rotten floorboards,
And if I ever dare to set foot inside
He’ll jump out and chop off my head,
But I’ll take a peek one day.
The third stanza of the poem introduces a new character: the speaker's brother. The poet's brother had told the speaker that there is a ghost inside the shed. The ghost is not visible to everyone's eyes as it is hidden under the floor. Also, the brother had warned the speaker that the ghost would jump out of its hiding place and pounce on him the moment he steps inside. Additionally, the brother said that the ghost would chop his head off if it sees him entering the shed.
There is a ghost in the shed, says the brother

The very idea of a ghost would probably have thrown the seed of fear in the speaker. Also, the shed’s appearance (the old, mysterious, and dusty look) would have built the same.

Again, the stanza ends with a modified refrain. The speaker is evidently scared of setting his foot inside the shed, so he says that would take a peep someday. Neither is he going to open the doors and nor is he going to look secretly through the windows. On the other hand, he will take a quick look, and more likely from a safe distance. Though the speaker is afraid, he also has the desire to see and explore the shed.
Meanings of difficult words:
RottenSomething that has decayed and cannot be used
FloorboardA long flat piece of wood in a wooden floor
Dare Have the courage to do something
Chop Cut something into pieces, often with repeated sharp blows of an axe or knife
Peek To look secretly and swiftly (very quickly)
National Council of Educational Research and Training (2007). Honeycomb. The Shed: Frank Flynn (pp. 48 -49). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.