The poem is about a father who presents himself as unafraid of anything, from monsters to ghosts, but the last line reveals he is actually afraid of spiders. In 3 sentences:
The persona sees his father as a heroic figure who is afraid of nothing, but the last line shows the father's real fear of spiders by having the mother remove one from the bath, revealing that heroes can have weaknesses even in everyday life. The poem suggests that while people may pretend to be fearless, it is better to be honest about one's limitations.