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Miniature Red Heart is an item added by Tinkers' Construct. These are extremely rare drops from all mobs, and one good way to get these is to have a mob farm and search through the large amount of loot from mobs that have been killed by it, until one is found. The fastest way is probably to use a soul shard and create a wither skeleton spawner. Using a decapitating axe you can quickly gather stacks of wither skeleton skulls to spawn withers, which drop miniature red hearts much more quickly.
They are found very frequently in Vanilla Desert Temples.
The Miniature Red Heart is used to craft a Heart canister with a Jeweled Apple and Necrotic Bone, which when used (in accessory slot) gives you one additional heart (2 HP). This boost is permanent, but has a cap of 10 extra hearts, using more will not add any more health to your player.