On the surface, this is a speaker dealing with a rat that has gotten into the walls of their home. They hear its quick, heavy footsteps, wonder how it got in, and try to coax it out with cheese and nuts. But the rat ignores the food, and the poem hints that something else is going on. The line about the rat preferring "my thoughts" suggests it stands for something inside the speaker's mind, such as anxiety, stress, or a worry that won't leave.
The speaker's mood shifts as the poem goes on. It begins with confusion and irritation, then turns tired and pleading: "I have suffered enough." The ending is quieter and more reflective. The speaker admits the walls are "weak and thin" and that this made it easy for the rat to settle in. That sounds less like blaming the rat and more like blaming their own vulnerability. The mood is uneasy, worn down, and a little resigned.