The title is the key: a shut-in is someone confined indoors, and the poem shows a person so alone that even their own body splits into two parties. One hand owes money to the other, as if the speaker is playing cards against themselves, keeping a debt that means nothing because both hands belong to the same person.
Then evening arrives. Night falls "on the cards of their house," a nice image that suggests a fragile house of cards, or a home reduced to a game. Nothing sturdy seems to hold it up. The last lines turn to a fish, or fish, moving back and forth in a tank, and the speaker compares their motion to "nonsense words." That fits the mood: repetitive, aimless, a little absurd. The tone is quiet and wry rather than openly sad, but the loneliness shows through. Time passes, the same small motions repeat, and the speaker watches without anyone to share it with.