Suppose you hold this hand:
   ♠ 9 8 5   K Q 10 8 3   K 3   ♣ A 10 4

and had this bidding sequence:
     1    1
    1NT   ?

You don't want to rebid your 5-card s and you know you must reach game so it is tempting to just leap to 3NT and hope for the best. However, if Opener happens to have 3 then a 4 contract should be better than 3NT. How can you find out if partner has 3 s?

In this bidding sequence, where partner opens 1 of a suit, then bids 1NT over your 1 of a Major response, the bid of a previously unbid minor by you is called New Minor Forcing. It does not mean you actually hold that minor, although you may, but it asks partner to further describe her hand. In particular you are asking if she holds 3 cards in your first (Major) suit. You know she does not have 4 of that suit or she would not have bid 1NT, but she might well have 3 of them. If she does hold 3 she should support the suit and if she has fewer she should find some other bid.

Lesson available.