A thing may be considered in two ways: as it presents itself as an object to our sensibility (thing as it appears) and as it is apart from its relation to sensibility (thing in itself).
B307: "The doctrine of sensibility is likewise the doctrine of the noumenon in the negative sense, that is, of things which the understanding must think without this reference to our mode of intuition, therefore not merely as appearances but as things in themselves. At the same time the understanding is well aware that in viewing things in this manner, and thus apart from our mode of intuition, it cannot make any use of the categories."
