William Blake's "The Poison Tree" is meant to teach us a lesson: when we are unhappy with someone, friend or foe, we should tell that person how we feel. If we nurse our anger inside, it will grow and eventually lead to sin and violence.
The lines you cited in the question are the last two lines of the poem. Before discussing them, let's review how Blake reaches that conclusion. The first stanza of the poem provides two contrasting scenarios:
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. (1–4)
When upset with his friend, the speaker tells his friend how he feels, and that puts an end to his anger (1–2). However, when he grows "angry with [his] foe," he keeps this to himself, and his "wrath did grow." Obviously, it's better to tell someone when you're upset with him, because if not, the anger will fester and corrupt the person who holds it.
The remainder of the poem describes how that anger grows inside the speaker until it "poisons" him completely. The second stanza introduces the metaphor
of the tree. The speaker's anger is now represented as that tree, and he "waterd it in fears" (5) and "sunned it with smiles" (7). The speaker's internalization of his anger and his feelings toward the foe work to nurture the "poison tree" inside of him.
The speaker takes the metaphor even further in stanza three when he writes:
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. (9–12)
Because the speaker allowed the anger to grow so mightily within him, it eventually "bore an apple bright." This description, along with the foe's recognition of the "apple," or the speaker's wrath toward him, indicates that the speaker's anger is now obvious to the foe. This leads to more direct action in the final stanza.
The speaker concludes the poem as follows:
And into my garden stole,
When the night had veild the pole;
In the morning glad I see;
My foe outstretched beneath the tree. (13–16)
Because the foe is aware of the speaker's anger, he has now come to confront the speaker. It is not detailed what exactly occurs, but in the morning, the foe is apparently dead under the tree. We can imagine he was killed by the speaker, and it's even more menacing that the speaker is "glad" about the outcome. The anger has turned into poison so strong it has led the speaker to commit a heinous act and to even celebrate it. The speaker has been utterly poisoned by his own wrath. The moral of the poem is that we should tell others when they've angered us; if we do not, we are risking our own ethics and morality in the process.