We know that opening 1NT shows 12-14 points. How, then, do we deal with hands that are balanced but either too weak or too strong to open 1NT?
The answer is simple enough. We open with a suit bid and then rebid in No-trumps. We bid them at a minimum level to show 15-16 points and we jump a round of bidding to show a hand stronger than this (that is: 17-18 points). With a whole 19 points we leap to 3NT.
So, f'rinstance, 1♣ - 1♥ - 1NT shows 15-16 points (and a balanced hand, naturally)
1♣ - 1♥ - 2NT would show 17-18 points.
1♣ - 1♥ - 3NT would show 19 points.
Having made your rebid you have put your metaphorical cards on the table. You have made a LIMIT bid and now it's up to partner. He can Pass, bid game or do whatever best suits him (and his hand). The ball is firmly in his court.
Let's see if this quiz makes everything clear. Crystal clear.