A parable is a short symbolic story that teaches a moral lesson. Blake's "A Poison Tree" is a parable about the dangers of suppressing anger. The poet explains the point he is trying to make in the first stanza, in which he says:

I was angry with my friend;
I told my wrath, my wrath did end.

In other words, when he became angry with his friend, he told him about why he was angry and he soon felt better, but

I was angry with my foe:
I told it not, my wrath did grow.

When he became angry with someone he disliked, he kept it a secret and  became more and more angry. The rest of the poem describes, using symbolic language, the consequences of not telling his "foe" of his anger:

And it grew both day and night.
Till it bore an apple bright.

His anger has become a tree that bears an "apple bright," like the Tree of Knowledge in the Garden of Eden. Suddenly the "anger" that started all this has become very serious, a kind of original sin. The poet, like the serpent in the Bible, tempts his foe with the apple, and his foe is too easily fooled:

In the morning glad I see;
My foe outstretched beneath the tree.

That is, his foe has eaten of the fruit of his anger, and is poisoned by it. The moral of this parable is that suppressing anger can make things much, much worse.