This distinction between phenomenon and noumenon is basically the same as the distinction between appearance and reality. The phenomenal and the noumenal are two aspects of The Real, viz., the aspect which appears to us when we perceive it, and the aspect that is actually really real. The physicists, for example, tell us that even though the chair appears to be impenetrable and solid, in fact it is made up of molecules and atoms which are themselves made almost entirely of empty space. So the way a thing appears to our senses may not be, at least according to physics, the same as the way the thing really actually is. The phenomenal aspect of a thing may be entirely different than the noumenal thing, the thing as it really truly is.
And Kant’s question is asking whether it is possible to know the noumenal aspect of things. It is obviously possible to know the phenomenal aspect of things, because the phenomenal is what shows, it is what appears to us (“phenomenon” is from the Greek word which means “to show”). But the question before us is whether it is also possible to know what is really real. Berkeley claimed that the only things that were really real were perceptions and minds, and Hume believed that everything we experience is really only a perception, and that what we think of as our self or identity is not really real at all.