Hit Points, abbreviated as HP, are your character's health. They are what keep you alive, and once they reach zero, you die.
Monsters or enemy players will reduce your HP on each successful attack. There are also damage over time skills which might be cast on you which drain your HP over a period of time. To gain back your HP, you can eat some HP restoring fruits (they are Red most of the time. There are also potions that can bring your HP back. Another way is to wait, as HP recovers with time. This process will be accelerated if you rest (press C on your keyboard)
You gain maximum HP when you level up. You can also gain more HP by spending Stat Pointsin the REC stat, which will increase your HP by 5 per REC point. Also, your equipment can provide you with some additional HP.