The poem is in the public domain, widely available in print and on the net, so here it is. The poem is in bold, my comments in plain font.
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.
Here Blake is setting the stage, and using the familiar idea that by discussing our anger, we can be rid of it, while by holding on to it it grows. I haven’t found this always to be the case. I can remember being really angry about things, never talking about them, and the anger gradually fading. But as a general rule, I think it’s correct.
And I water’d it in fears,
Night & morning with my tears;
And I sunned it with my smiles
And with soft deceitful wiles.
Now Blake starts getting poetic, using imagery and metaphor. He has not yet introduced the tree of the title, but he is hinting more strongly at it, moving from the very abstract “growth” of the first stanza to a more concrete image – something that can be watered and sunned – but it is not watered with water or sun, but with fears and tears, smiles and 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,
The tree is explicitly introduced – indeed, it is a specific kind of tree, an apple tree, which resonates with the story of Adam and Eve. The apple is shiny. And Blake’s foe knows that it belongs to Blake. When hearing this poem aloud in my head, I always imagine a stress on that last word “mine”. Apples are easy to get – but the foe’s apple has a special appeal.
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
In this final stanza everything is concrete. The apples are not (seemingly) metaphorical anymore – the foe has stolen them, eaten them, and died. But yet things are metaphorical still – could such a tree really be grown? No. So, we have to make the connection in our heads – these really are not real apples. But the foe is nonetheless undone by his own enmity. Blake doesn’t kill his foe, he simply lets the foe kill himself.
And Blake manages to do all this while sticking to a strict rhyme and meter, and with none of the rhymes seeming forced. Well, there are reasons he’s one of the master poets of the English language.