In "A Poison Tree," Blake is comparing his wrath, which means anger, with the nurturing of a tree. This is what we would call an extended metaphor, since he carries it throughout the entire poem. We can nurture our emotions, as we can nurture a plant, and the poem shows us a contrast of what happens when we do not nurture our anger and what happens when we do.
The first two lines tell us that if we are angry at someone and we let them know, we are not feeding our anger. When you say what is on your mind and deal with it, it is over and done with. But when you are angry and nurse your anger, it is going to grow, just as a tree that is fed and watered and given sunshine is going to grow. That is what the second verse is saying.
The third verse is telling us that nurturing our anger produces a kind of fruit, just as a tree that is nurtured can produce an apple. When we hang onto our anger like this, in some way we find it pleasurable. We actually feed on it ourselves.
The last stanza finds the "foe" attracted to the apple on the tree, and he steals into the garden, eats the apple, and dies.
To be honest, I find this part of the poem unsatisfactory because, to me, the message one should take away is not that holding onto your anger can poison your foes, but that holding onto your anger can poison you. I find that to be a greater psychological truth. People who hold onto their anger can really poison themselves, to the degree their anger can kill them, with heart attacks and strokes, for example. However, who am I to quarrel with Blake, who really was a very fine poet and artist!