Two short lines carry a whole day of tired, repetitive effort. The title, "Treadmill," sets the scene: a machine that keeps you moving without taking you anywhere. The speaker then compares themselves to a rat, the kind of creature we picture running in a wheel, and also says they work "as a blind heart."
That last phrase is the strangest and most interesting part. A heart does constant, necessary work, beating all day without a break, but it can't see where it's going. Put the rat and the heart together and you get a sense of labor that is steady, driven, and a little hopeless. The speaker keeps going, but without direction or a clear view of the point.
The mood is weary and quietly bleak, though the poem is oddly intimate too. Addressing the rat directly ("o rat") makes it sound like the speaker is talking to a fellow sufferer, or perhaps to themselves.