What is the meaning of this poem? ‘A Poison Tree’

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

And I water'd it in fears,
Night and morning with my tears;
And I sunned it with smiles,
And with soft deceitful wiles.

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 veil'd the pole:
In the morning glad I see
My foe outstretch'd beneath the tree.