A 3NT opening is "Namyats" — it shows a good preempt in one of the majors. It is forcing: responder is not allowed to pass it out. Almost always responder answers with either 4♣ or 4♦ to sort out which major opener holds. The other responses below are specific asking bids that opener created long ago; they are recorded here for completeness, but in practice they essentially never come up.
4♣ Transfer opener to their major. Cuebids and RKC available after. 4♦ Ask opener to bid their major. Cuebids and RKC available after. 4♥ Asking bid. Asks for opener's suit and control in the OTHER major. Passable. 4♠ Asking bid for ♣ control. 4NT Asking bid for ♦ control. 5♣ Asking bid for ♣ high-card control only. 5♦ Asking bid for ♦ high-card control only.
For the asking bids, opener replies by stepping up the line, alternating ♥ then ♠, so that each step names both the suit and the level of control.
Over 4♥, the steps are: Pass = no control, then singleton or void, then high-card control. Because Pass is the first step, opener can stop in 4 of a major when there is no control in the other major.
Over 4♠ and 4NT, the steps run: no control, singleton or void, high-card control (alternating ♥/♠ as above).
Over 5♣ and 5♦, the same scheme applies but with no singleton-or-void step, since only a high-card control counts.
3NT–4♥–Pass shows hearts with no spade control. This lets the pair stop in 4 of a major when responder is interested in slam but cannot yet tell which major opener has and there is no control in the off major.
3NT–4♠–5♣ shows spades with no club control.
3NT–5♣–5♠ shows hearts with a high-card club control.