This addon adds heart containers that can be crafted from 9 heart shards. Heart shards can be found in their ore form underground. When you hold a heart container it gives you 2 permanent hearts.
When you hold the heart container (recipe below) you will get 2 more hearts. If you die, you will have to use another heart container to get your hearts back. To get 4 rows of hearts, you need 135 heart shards. The ores spawn underground near the iron level and is pretty common.