This is a very interesting poem that has a thought-provoking moral at the end of it. Basically, the speaker of the poem describes two scenarios: the first is where he was upset with someone, told that person about it, and then the incident was over and done with: "I was angry with my friend:/I told my wrath, my wrath did end." So, the point here is that when you are upset with someone, it is good to talk about it, to end the matter and move on. The second scenario he describes is when he was angry with someone and kept his wrath within him instead of speaking about it; as a result, it "did grow". He thought about his anger more and more, and he nurtured it. He "waterd it in fears," dwelling on his anger and fears, and tried to hid it as he "sunned it with smiles." The wrath becomes so large and noticable that Blake uses a metaphor of an apple growing from a tree to describe how it feels to him; it is an apple poisoned with his wrath. This is symbolic of how our anger can only yield bitter and poisonous results (or fruit) that are no good to anyone. Along these lines, at the end he states that
"In the morning glad I see;/My foe outstretched beneath the tree,"
which seems to indicate that the foe has partaken of this poisoned apple, and has died. If your wrath and anger for someone is nurtured within you for so long, it eventually turns to poison, and that poison often does great damage to the person you are angry with. It's kind-of like bottling up rage and having it explode, with disasterous and regretful results, instead of just dealing with your anger right away. Blake uses a poisoned apple as a metaphor for that toxic anger that the reader grew by dwelling on it, and his foe, outstretched on the ground, is a symbol for the speaker having released his anger finally, and how it completely destroyed the other person. I doubt Blake is referring to actual murder, but think of the consequences of blowing up at someone; it ruins friendships, self-esteem, families, marriages, and often has lasting impacts. Blake's moral is that when we are angry, we should, as he did in line 2, talk about it and get it out there, so that it can "end" in a better way.
I hope that explanation helps a bit; good luck!