Question

What are Sweet Hearts?

Answer 1

Sweet Hearts can...

1. Restore a Pokemon's HP by 20 points
2. Be sold to Route 5's gourmet maniac for $100
3. Be traded to a woman in Mistralton City in exchange for a Heart Scale
   • Typically 10 Sweet Hearts required, but only 5 in BW2 on February 14 and March 14

Sweet Heart (Bulbapedia)

Answer 2

"A very sugary chocolate. It restores one Pokemon's HP by 20."

It's basically a recovery item introduced in Generation 5. However, it can apparently be traded for other interesting things. We don't know much since the game hasn't come out yet.