According to Kant, his way of understanding the relation of objects of knowledge to the mind was essentially a metaphysical thesis that distinguished between two divisions of objects: the appearance of the objects and the objects themselves. The things in themselves, in his explanation, are completely real in the way that they would exist and have the same properties regardless if human beings never existed and never around to observe them. In contrast, the appearances of the objects were the opposite of real because the existence of appearance and the visual properties depend on human perception, and perception, in a sense, exists in the mind.
I feel that Kant compared his philosophy to the Copernican Revolution because of the similarities. Because the Sun revolving around the Earth was impossible to explain, Copernicus adopted an alternative approach. Copernicus hypothesized that it was the Earth that revolved around the Sun. Kant took a similar approach philosophically and felt that the perception of objects didn't move around us but that perception revolves around the objects themselves.