We know that opening 1NT shows 15-17 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 with a minimum hand to show 12-14 points and we jump a round of bidding to show a hand too strong (that is: 18-19 points).
So, f'rinstance, 1♣ - 1♥ - 1NT shows 12-14 points (and a balanced hand, naturally)
1♣ - 1♥ - 2NT would show 18-19 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.