From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sweet Heart (Japanese: ハート スイーツ Heart Sweets) is an item available in Pokémon Black and White.
In the games
Sweet Hearts are obtained through use of the C-Gear's Feeling Checker. They restore 20 HP to a single Pokémon when used. A woman in Mistralton City will exchange ten Sweet Hearts for one Heart Scale.
Sweet Hearts are based on heart-shaped chocolates which are a popular gift between lovers.