When your partner opens 1NT, you know almost exactly what kind of hand he holds. It is one of only 3 possible shapes: 4333, 4432 or 5332 - the balanced hands. And it sits in a very narrow point range: 12-14 if you are playing Acol; or 15-17 if you are playing Standard American.
Responder’s 1NT, often called the “Dustbin 1NT”, often causes confusion in bridge learners. Many people remember the bid to mean “6-9 HCP”, “no fit in Partner’s suit” or “any old rubbish”. The truth is that these are all part of the answer: but not the whole answer! The bid should not be thought of in terms of what it shows but what it denies. It simply means “I couldn’t do anything else”. No wonder it’s so difficult to get your head around!