Theory:

And it grew both day and night,
Till it bore an apple bright.
And my foe beheld it shine,
And he knew that it was mine.
 
And into my garden stole,
When the night had veiled the pole;
In the morning glad I see,
My foe outstretched beneath the tree.
Explanation:
 
We learn about the outcome of the poet’s hidden anger, symbolized by an apple treeAs we all plant a tree and take good care, it grows and bears fruit. The poet compares the anger he grew within himself day and night to the shiny apple fruit from the poison tree that he grows. The poet's enemy saw the apple fruit, which was shining, and he knew it was the poet's.
  
apple-3066196_1280.jpg
  
We human beings steal fruit from a garden and eat it without understanding the consequences. In the same way, one day the poet’s enemy fell into the trap and gets into the poet's garden during the darkness covered the pole star and stole the apple from the poison tree and supposedly eats it in the night, as the speaker observes him gone the following day. What this denotes is the speaker’s anger eventually coming out. The wrath has built up to such a point that it cannot just be told; it must be acted upon. And acted upon it was. The speaker saw his enemy dead, and he was pleased at the sight.
  
Meaning of difficult words:
 
Numbers
Word
Meanings
1.
VeiledSurrounded.
2.
Beheld Observed.
3.
Outstretched Go beyond limit.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. A Poison Tree - William Blake (pp. 182-183). Published by the Tamil Nadu Textbook and Educational Services Corporation.