Each time a player rebids his suit (without Partner raising it) he shows 1 card more in that suit than promised in his previous bid.
Opener's 1♥ opening bid shows at least 5 cards in the suit. His rebid of 3♥ now shows 6 hearts and also 16-18 points.#

East, with 2 hearts and 10½ points (2 shortage points), can now raise to 4♥.

After winning the opening lead with the ♠A, what does Declarer do next ?

# = With 6 hearts and only 13-15 points Opener would have rebid 2♥.