As an unpassed hand, a 2NT bid over a 1♥ or 1♠ opening is a game-forcing raise (13+ dummy points, 4+ trumps, generally balanced and/or extras) that asks opener to describe his hand to help responder evaluate slam prospects. For example, over 1♥:
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3♣, 3♦, 3♠: singleton or void in that suit.
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4♥: minimum balanced hand.
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3NT: medium hand (16-18 total points).
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3♥: maximum hand (19+ total points).
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4♣, 4♦, 4♠: 5-card suit with two of the top three honors.
This treatment is also on after a takeout double but is off as a passed hand (see below).