The power stored using the move Stockpile is absorbed by the user to heal its HP. Storing more power heals more HP.
This move has four tiers of healing; 3 are stockpiled, 1 isn't. Unfortunately the one that isn't is that the attack doesn't do anything if you haven't stockpiled so you have to make sure to Stockpile first. If you use Swallow after 1 Stockpile, you will recover 1/4 of your Max HP If you use Swallow after 2 Stockpiles, you will recover 1/2 of your Max HP If you use Swallow after 3 Stockpiles, you will recover all of your Max HP