Health is the amount of damage you can take before dying. The amount of health you have at the beginning of the game is half of the sum of your Endurance and Strength (excluding any bonuses to the two from birthsigns). When leveling up, your health is increased by 10% of your Endurance. You can restore your health with Restore Health potions, enchanted items, spells, or by resting. You can check your health by looking at the red bar in your menu or at the bottom left of the screen (as seen in the image below).
Health can be damaged by hits from weapons, fists, several types of spells, falls from high places, lava, or running out of breath while underwater. If you lose all your health, you die, except in one case that may come about while exploring the Puzzle Canal in Vivec.
Weapons and armor also have a "health" value listed as "Condition". This value is shown in the item info and as a red bar underneath the icon for a weapon. Weapons with low Condition will do less damage, and a weapon with zero Condition cannot be used at all until repaired. Armor with a low Condition provides less protection, and armor with a Condition of zero cannot be worn.