when your HP reaches 0, you die. And unknown power summons your soul.

Request Monk for resurrection.
This gives you a great side effect.

During the journey through the world of 4 story gold, you will face numberless challenges not only from the monsters but also from warriors from the hostile Kingdom.
Marching into the perilous adventure with lots of dangerous factors, you will face the situation where your HP reaches 0 and die. This is not the end of your adventure. 

Chaos Monks are spread out through the map including each village. Chaos Monks are the only ones who can detect your soul. Using their unique magical power, they detects the souls wandering and bring them back before the souls get sucked into Avalon. Once the soul has been brought back, the soul is granted with another chance to resuscitate.

The soul has to find its body in order to resurrect and when it does, you will suffer from a side effect of losing some of your HP for certain period of time. This is why you need to be alert of you HP/MP gage at all times and try not to die. HP/MP potions can be a great help to keep you alive.

If you are not in a situation where you can find your body. You can also right click on the monk and click on 'Resurrection'. This will bring your body to where you are, that way you don't need to go back to where you died. However, this will bring a serious side effect of losing HP/MP.

Likewise, if there is a Priest near the body, you can resurrect with the help of Priest.