This is the text of the poem (provided by Bartleby.com):
Part Four: Time and Eternity
I SHALL know why, when time is over,
And I have ceased to wonder why;
Christ will explain each separate anguish
In the fair schoolroom of the sky.
He will tell me what Peter promised,
And I, for wonder at his woe,
I shall forget the drop of anguish
That scalds me now, that scalds me now.
In this poem, the narrator is addressing the issue of suffering and how no matter what she suffers now, it will be all worth the glory she will see in Heaven once she dies.
She uses several comparisons here. First, in the first stanza, she compares God to a schoolteacher. She says that God will tell her why he had to put people through difficulties in their lives. The narrator is happy that she will have answers, no doubt, when that time comes.
She also compares the difficulties and painful times she has encountered to scalding liquid when she says that the woes sha has experienced scald her as hot water or hot coffee might.
These comparisons work well because they make something so abstract (religious beliefs and the vision of Heaven, for example) very concrete, believable, and common to humankind.
In response to the last response, yes, I do believe there could be irony in this poem or that the whole poem can be ironic; however, I tend not to believe that it is based on my studies of other Dickinson poems tha are similar to this one. You very well may be right, however, about any interpretation that might be too simple.
This is more a questions than an answer. Does anyone suspect that this entire poem is ironic, that she does not mean it as it reads on the surface? In the Dickinson canon there are many poems that speak of the inscrutibility of human suffering and the distance of God --- how could this poem suggest such a simple "answer"?