Suppose opener begins with 1H. Responder may have a weak hand with 5-card support (6-AT865-Q54-JT32) and the right bid is 4H. At other times, responder may have the "real thing" (6-AT86-AJ54-KQJ2). It can not be right to bid 4H with both hands, so many pairs employ a 2NT response to 1-of-a-major to show:
– Game going values (13 + tps), and
– 4+ card support
Or
– Better than game values (16 + tps), and
– 3 card support
That way opener can reasonably assess prospects for slam. The 2NT response also keeps the auction low enough for the bidding side to discuss their assets further before reaching the 4-level. Of particular importance for trump slams is the location of shortages. Here's a widely played structure:
O: R
1S: 2NT
3C = club shortage (singleton or void)
3D = diamond shortage
3H = heart shortage
3S = non-minimum, no shortage
3NT = Balanced maximum (18-19)
4C / D / H = 5 card suit
4S = minimum, no shortage.
Responder can assess her holding in opener's shortage and cue-bid to encourage slam or sign-off in 4 of the major to discourage. For example, South holds:
KQ42
9854
Q6
AK3
And hears partner open 1S. She bids 2NT, showing support and game values and North bids 3H, showing a shortage in hearts. With no wasted values in hearts, South can cue-bid her club club ace to encourage partner towards slam. By contrast, If South held:
AK43
KQ54
Q6
765
And the auction began the same way: 1S: 2NT: 3H (shortage), South's heart values seem wasted and to say so, South can sign off in 4S.