Your hit points (HP) reflect your character's remaining life. HP lowers whenever you take damage from an attack or effect. Should it ever reach 0, you pass out and begin making death saving throws. If you fail 3 death saving throws, you die.
Your HP is primarily determined by your class. Each class description specifies how much HP it gets at first level, and how much its HP increases with each additional level. Your Constitution score (not modifier) gets added to your HP, and whenever your Constitution increases your HP increases by that amount too.
Lost HP can be healed by various powers and potions, and by spending healing surges.