Opening the bidding with 1NT shows a special hand type. It's balanced, with 15, 16 or 17 points. There are three balanced hand patterns: 18.104.22.168, 22.214.171.124, and 126.96.36.199 You don’t need high cards in each suit to open 1NT, although that would be perfect.
Here's a typical 1NT opening: ♠ AKJ4 ♥Q1032 ♦KJ10 ♣K7 (17 high card points, and 188.8.131.52 shape).
Your partner is saying a lot about their hand when they start with 1NT. As responder, add your points to 15,16 or 17. If you have a long suit, bid it. The 1NT opener will have at least two cards in your suit, because their hand is balanced. If you have a balanced hand, stay in no trumps. If you have an unbalanced hand, choose your longest suit to be the trump suit. The weaker responder is, and the longer their suit, the more that suit should be trumps.
If you bid up to a level known as game, and make your contract, you score extra bonus points on top of your trick score. It’s 300 or 500 extra depending on “vulnerability”. Your partnership needs 25+ points to bid to game.
Game: 3NT, 4 ♠ or 4 ♥, 5 ♦or 5 ♣
3NT has the fewest number of tricks required for game, so it’s popular.
0 – 7 pts, pass (1NT p p )
8 – 9 pts, raise to 2NT (1NT p 2NT)
10+pts, raise to 3NT (1NT p 3NT)
0 – 7 pts, bid 2 of a long suit (1NT p 2♦/♥/♠)
8 – 9 pts, bid 2NT (1NT p 2NT)
10+ pts, jump to game with six+ cards (1NT p 4 ♥/♠) , or to 3 of your suit with five. (1NT p 3 ♥/♠).