Dealer opens 1D and Partner responds 1H. Dealer then rebids 2NT. What does Dealer's 2NT rebid mean? Opponents have passed throughout.
Playing a system such as Standard American this shows a balanced or semi-balanced hand with 18-19 HCP. It would usually deny holding 4 Hearts unless with a 4333 distribution, though partnerships playing some form of Check-Back Stayman may relax this. One might look for an alternative sequence with a holding worse than Qxx in an unbid (Clubs and Spades here) suit.
The rationale is that it is covering the range too weak for an initial 2NT opening bid, now usually played as 20-21 HCP, and too strong for either a !NT opening (12-14 if weak and 15-17 if strong) or a 1NT rebid (the ranges just mentioned reversed).