Richard Wilbur's poem "Love Calls us to the Things of This World" describes the state between wakefulness and sleep, the moment someone is just starting to wake up in the morning, which is also a state that's halfway between the spiritual world and the corporeal world. In describing the spiritual world, Wilbur progresses to describe angels while also creating an interesting metaphor that relates angels to laundry. Angels are the ones who are dressed in the bedsheets, blouses, and smocks hanging on the clothes line.
By the final stanza, the person in the poem has fully awoken to remember "the punctual rape of every blessed day," meaning all of the tribulations typically associated with one human day. From there, Wilbur continues to describe all of humanity dressed in the holiness of clean laundry, even the thieves.
He ends the final stanza by describing the "heaviest nuns" dressed in "dark habits" as they "walk in a pure floating / Of dark habits, / keeping their difficult balance." These final three lines describe something both corporeal and spiritual.
Due to the length and flow of the habits that nuns wear, when they walk, they literally appear to be floating. The stereotypical nun is also rather obese; hence, the phrase "keeping their difficult balance" can literally refer to obese nuns trying to remain gracefully on their feet as they try to walk in their habits in a way that appears like they are floating. In other words, the phrase "keeping their difficult balance" can literally refer to the nuns physically trying to remain standing and walking despite their weight and the awkwardness of their clothing.
But we can also give the phrase a spiritual interpretation. The color black represents darkness and often symbolizes the sin of this world. Nuns are spiritual people, but they are also still people. Therefore, nuns have a duty to remain balanced between the sinful state of the corporeal world and the spiritual world; they must embrace the spiritual world while still being burdened with their sinful state as corporeal human beings. Hence, "keeping their difficult balance" can also be referring to how difficult it is for nuns to maintain this spiritual balance, to remain holy while still being sinners.