The speaker describes how he bottled up his wrath for his foe. Over time, his wrath increased. In the second stanza, he says he watered it "Night & morning with my tears, / And I sunned it with smiles." He means that he allowed his wrath to grow, day and night. The third stanza continues this notion. His wrath grew into something more. He notes that it grew (metaphorically) into an apple. This metaphor is also an allusion to the fruit which Adam and Eve ate in the Garden of Eden. This is a reference to humanity's "fall" into sin. Blake uses this allusion to connect the speaker's wrath with the wrath that God showed Adam and Eve in this story from the Old Testament.
In the last stanza, "night veiled" the sky. The "pole" possibly refers to the North Star and this means that the "pole" generally means the sky. So, the night is dark and perhaps foggy. It is during the night that his foe crept into the garden, the place where his wrath has grown. Night is often associated with darkness and evil and these are logical symbols that describe the place and time in/during which "wrath" thrives.
The last few lines are a bit ambiguous. If "glad" describes the morning, it would mean that the morning is bright. In this case, he may ("in the light of day") have regret that his wrath has killed his foe. Or, "glad" describes "I," the speaker. And in this case, the speaker is glad to see his foe dead. This is probably the correct interpretation because Blake is condemning one who cultivates wrath: the speaker's wrath as well as the idea of a wrathful God.