Conventions in depth
An opening 1NT shows a balanced hand of 15 to 17 high-card points.
Balanced means no singleton or void and no more than one doubleton, so the shape is 4-3-3-3, 4-4-3-2 or 5-3-3-2 (the five-card suit being a minor, or a major not worth opening one of).
Because the picture is so exact, responder can add the two hands together and judge game or slam to within a trick, and several responses exist to find out more:
The other balanced openings: 2NT shows a balanced 20–21, and the very strongest balanced hands (22 or more) open an artificial 2♣ and rebid notrump. A balanced hand below 15 is opened with one of a suit and shown with a notrump rebid on the next round, or passed.
The strong 1NT is not so much a convention as the basis of the system: a limited opening that Stayman, transfers and quantitative raises are all designed to explore.
