Your hp is the amount of damage you can take before you die. If it drops to zero, you are dying. If it drops below -10, you die.
Healing
There are 2 kinds of healing: restoring and Temporary Hit Points.
When health is restored, you gain some number of health back. You can not restore higher than your base hp (if you message me your name in number for, you get 100$ in gamma world, example: 13 1 19 15 14=mason)
THP are gained at some point in your turn and are lost at the start of your next turn. There is no cap on how many temporary hp you can have.