Health is how much damage you take take before passing out, then dying. Pretty simple concept.
When you hit 0 HP you aren't quite dead or dying yet. You're still awake, but you are unable to take more than one action per round. If you make any strenuous action, such as attacking an opponent, you lose 1 HP and are now bleeding out.
From -1 to -9 HP you are bleeding out. Your character is unconscious and can make no actions. Every round you are bleeding out, you take -1 HP. You can make a Fortitude check (DC 15) each round to see if you stabilize naturally. If you stabilize, you no longer lose 1 HP per round.
At -10 HP, you die. In Runescape, not even the gods can bring back a soul claimed by Death. Therefore, Death is permanent.
If you are able to get to a dead character who has not died of grievous injuries and exsanguinated (An example of a suitable situation: Dying of shock damage, drowning, or anything else that leaves the body in tact.) within 10 rounds (1 minute), mages can make a Magic Check (DC 25) to restart the heart with a shock spell. Non-mages can made a Reflex Check (DC 25) to apply CPR to the dead character.
In either situation, the dead character must make a Fortitude Check (DC 25) to revive. If they roll a Natural 1 on the Fortitude, they immediately and permanently die. The Magic, Reflex, and Fortitude checks can be taken as many times as they can until 10 rounds (1 Minute) after the character has died. If they fail to revive the character by the 10th round, they die forever.
If you have been revived from your death bed, the situation has scarred you. You lose 1 character level (Along with any benefits it brought) and are now Marked by Death. Next time you hit -10 HP, Death rides for you swiftly and your soul is claimed immediately.
Wights are tormented souls bound to their bodies and twisted to the will of their new master. For whatever reason, should you successfully create a Wight (Magic DC 35), it uses the character sheet of the character subtracting two levels and all its benefits. They act as a follower for you, and do not have free will.
You can recover health in many ways. Naturally, you regain 10 HP per real-life day. Certain spells and potions can help with more immediate healing and have their own special rules.
Medicine supplies can be used to heal 1D10 damage on a character. However for each 1D10 you heal a character, a day is subtracted from how long the medicine supplies would last. So if it were slated to run out on 3/23 and you heal a character for 3D10 with those supplies, the new day it runs out is 3/20.