The poem does not explicitly tell readers what happened between the narrator of the poem and his friends. All that we know is that he is angry with a friend on two different occasions. The narrator tells his first friend about his anger, and the anger and hurt feelings go away; however, he does not tell the second friend about his anger. Consequently, his anger and hurt feelings grow.
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.
The second stanza explains that the poem's speaker nurtures his anger and makes it grow. He does this by "watering" it with his tears and fears. He also gives his anger plenty of nurturing sunshine. Obviously the narrator is not actually doing those things to his anger. The stanza is a metaphor
that compares the anger to a plant. You take care of a plant by watering it and letting it get sunshine. The speaker is caring for his anger in order to make it grow and grow.
And I watered it in fears,
Night & morning with my tears:
And I sunned it with smiles,
And with soft deceitful wiles.
Thematically, the poem is about the dangers of anger. More specifically, there is great danger in harboring hurt feelings and anger against people. In the first stanza, the speaker talks about his anger, and the friendship continues. In the rest of the poem, the speaker doesn't resolve the issue, and he ends up killing his friend.