The speaker pictures revising a piece of writing as a kind of entering, as if the manuscript were a room. The first lines make a striking claim: going in to change the text will change the writer too. Editing here is not just fixing words but reshaping the self.
The second stanza turns to something plain and bodily. A poor dinner sits "easy in my stomach," which suggests a modest, unfussy contentment. The speaker seems to need little, and perhaps writing feeds them more than a rich meal could.
The last stanza shifts again, into "a king's house / of melancholy cards." The image is hard to pin down. It could suggest a grand but sad place, or a fragile house of cards, or the weight of fortune-telling. Judging by the imagery, the manuscript may be a place of both power and sadness. The mood is quiet, spare and a little wistful, with meaning left open for the reader to complete.