"A Poison Tree" describes a series of events - it tells a story. Explain your understanding of the story’s significance.

This "story" is about the narrator getting angry with an enemy.  He keeps the anger to himself, allowing it to get more and more intense.  He lets is "grow" inside his body.  Symbolically, he plants a tree of his anger in his garden, and it grows to bear an apple.  In the story, the enemy eats the apple and it kills him.  The connection is that the narrator's anger has born a deadly fruit that will have negative - even murderous - consequences on his enemy.

Blake is trying to show how dangerous wrath can be.  The wrath the narrator has towards his friend is spoken of and quickly disappears.  This is what is supposed to happen.  However, the wrath towards the enemy does not get released, and it brings with it much negativity.  The idea that the wrath brings forth a poison apple is symbolic of the Garden of Eden.  It was human sin in the garden that caused Eve to eat the apple and get her and Adam kicked out of the Garden.  Blake uses the apple to suggest that wrath/anger is a sin and needs to be put aside, not let to grow.

The narrator isn't the only guilty party, however.  The enemy also sins:

And my foe beheld it shine,
And he knew that it was mine,
And into my garden stole

Here, it is the sin of "coveting" that causes the enemy to want what is the neighbor's.  By taking it, he is just as guilty in his own death, and proves again that humans share original sin.