Most popular esports titles have different types of modifiers to health and Overwatch is no different.
Overwatch has four different types of health modifiers that can affect you in certain scenarios. Regular health, armor, shields, and barriers all work on any hero and each have a different effect.
Just like how it sounds, health is, well, your health. The white notches in your health meter is your basic hp resource. All damage done to regular health does full damage. Health can be refilled from any means of healing; health packs and certain hero abilities will restore health.
Every hero has a certain amount of regular health.
Mercy is the only hero that can self-regenerate while others need to use abilities or passives.
Shown by the yellow bars in the health meter, armor halves incoming damage. However, you can run out of armor if you take enough damage to destroy your yellow armor bars. Innate armor, armor that certain heroes spawn with, can be healed. However, armor that has been picked up from Torbjorn's Health Packs cannot.
Heroes with armor: Bastion, D.Va, Reinhardt, Torbjorn, and Winston.
Shields are added to your health meter with the light blue bars. Shields act exactly like regular health - they won't reduce damage taken, but they do restore 30 points of shield per second after being out of combat for three seconds. So shields regenerate and health doesn't.
Heroes with shields: Symmetra, Zarya, and Zenyatta.
The only abilities in Overwatch that will fully block damage, barriers are extremely crucial. Only certain heroes have barriers that they can use, and each of these can block a certain amount of damage before breaking.
Heroes with barriers: Zarya, Winston, Reinhardt, and Symmetra.
Compositions aren't entirely dependent on having a mix of all of these types of armor, but usually, games go smoother when these modifiers are all applied. Pro teams engage with certain carriers while barriers or armor have been applied. You should try it out and stop playing Hanzo.