The speaker of Blake's "A Poison Tree" describes getting angry at a "friend," but then he tells his friend how angry he is and the anger goes away.  Then, the speaker gets angry at a "foe," but he is unwilling to tell his foe about his anger.  Therefore, his anger builds until until he explode with wrath, and the foe is found beneath the poison tree, dead.

The main theme of the poem is that anger left unchecked or suppressed can be cultivated into something much more destructive and disastrous, a "Poison Tree," the extended metaphor.  The Garden of Eden imagery suggests that the poem is a kind of morality tale.  The speaker personifies his anger as a tree, which he "waters in fears" and "suns it with soft, deceitful wiles."  The tree bears fruit: an apple, which is a symbol of anger.