The green health bar on top, and the blue mana at the bottom.
Health (commonly known as HP, itself meaning Health/Hit Points) is the amount of life a hero has. It is represented in the interface as a green bar with two numbers ( # / # ): the first one represents the actual life available at the moment, while the second is the maximum amount of health the hero can have. If a hero's health reaches zero and he doesn't have any abilities preventing it, death will occur. Health scales additively, meaning that each bonus point acquired directly affects the statistic. The blue bar below the health bar is the mana bar used for casting abilities.
Health can be increased by increasing the amount of strength that the hero has through items or stat leveling, as well as buying items that grant a flat bonus to health. Additionally, heroes will automatically increase their health every time they level up, the amount increased is dependent on their natural Strength progression.