What is the poem "A Poison Tree" mainly about?

"A Poison Tree" examines the effects of unresolved anger. In the poem, the narrator or persona first gets angry at a friend. He talks to his friend and that dialogue resolves his problem so that he can forgive his friend. Then he grows angry at an enemy. Rather than communicate with his enemy and hash the problem out, he holds the anger inside. As the poem puts it, he "waters" the anger with his tears, and then "suns" it with the false smiles he offers his enemy. Eventually, this anger grows and grows until it becomes a tree that bears a shiny, poisonous apple. The enemy eats the apple and dies. The poem might remind the reader of the apple that the serpent offered Eve in the Garden of Eden. The poem conveys the message that unresolved anger that is nursed as a grudge becomes a poison that can hurt other people.