If you hold 15-16 points and a balanced hand, you will open the bidding with a bid of 1 of your longest suit with the aim of rebidding 1NT if your partner makes a 1 of a suit bid.
But suppose your partner makes a bid of 1NT. What should you do then?
A rebid of 2NT would show 17 – 18 points. Your partner has already made the bid you intended to make and as they haven’t made a higher bid you know that they have a fairly weak hand. You should pass.