The basics is that your strength stat gives you more attack power. Attack power is calculated as damage. Something like 50 attack, will do 100 or so damage. Endurance will give you more defense points, and makes you able to block about that much damage. As HP decreases, you become more tired and the more tired you are, the harder it is to concentrate enough to perform attacks, persona summoning, or persona abilities. The key is to try to minimize on damage, or be able to heal or be able to be healed. If your health is reduced to zero, you fall unconscious. If you're attacked again while unconscious, you will go into a death-like state. Death-like states are the end, and there is not much that can be done to revive you. Unconsciousness can be recovered if an ally uses a revival item or the skills Recarm or Samarecarm. A Death-like state is a bit more difficult to solve. It can only be recovered through a Balm of Life or Samarecarm, and that still leaves the revived person with near zero health. If the person is not revived in the next two post, they are gone. Unconscious players may stay unconscious for as long as necessary. So long as no harm befalls them, they're safe. You must be very cautious with your health. Always have allies, and always be sure both you and your allies have revival items.
It should be noted that if you do die, an administrator will talk to you and will negotiate a term of "reincarnation". The most basic would be to have you create a new character entirely, a new persona, and give you the levels you had before on them (thus giving you all the abilities your new persona would have to that point). Stats would be randomized based loosely on what they were before, however. However, all money and items are lost and any alliances will need to be reforged. It is a new character, after all, and they won't immediately know how to use their persona either. If it was evolved, the the new persona will be evolved too. Can be a hefty loss, but this is the most basic of reincarnations.