This item is found only in the mobile version(s) of Terraria.
The Heart Arrow is a type of ammo exclusive to the Mobile Version of Terraria during Valentine's Day, when they are purchased (along with the Valentine Ring) from the Merchant. They can also be found inside a chest in a heart shaped cave in the snow biome.
Enemies hit by Heart Arrows will be stunned.
With its stunning ability, these are a viable choice of weapon against the Dungeon Guardian and bosses with a single hit-box, like Ocram.