The systems in Monster Hunter Wilds can be overwhelming at first, but over time you'll realize how helpful the UI is. In this beginner guide we'll be covering the most important Health Bars you should know.
"You're hungry" = Incomplete Health Bar
Hunting or exploring on an empty stomach is a recipe for disaster. Above, you can see a significant chunk of your possible max health is greyed out.
By making a habit of eating a meal before you venture out into the world, you'll save yourself a lot of trouble later on.
"You've taken some damage" = Partially Red Health Bar
When you've taken a hit, you'll see a red bar appear. This represents the amount of health you'll gradually recover back to if you stop taking damage.
If you regain poise and control of your fight, you can conserve your 
Potions and other healing resources.
"You're healing" = Partially Red and Greyed Health bar
If you do end up needing and taking a Potion, your health bar will indicate how much you will heal with a grey bar.
So if you're very low and see that even with the grey bar you're far from health, consider queueing up another one quickly with your Radial Dial.
"You need a Nulberry" = Colored Health Bar
When you're affected by a negative status effect, like 
Fireblight or 
Poison, your health bar will correspond to that related color.
In this case, be ready to use a 
Nulberry before using a Potion.
As another side note, you can see all the status effects you're affected by (both positive or negative) at the top left of your health bar.
It's obvious with this zoomed in screenshot but can be easy to miss when in full screen.
"You're going to die if you don't do something!" = Orange and Flash Health Bar
Last but not least is your Spidey - I mean, hunter survival sense that let's you know when you NEED to avoid the next attack.
If you can't take cover or reliably roll away, call upon your trusty Seikret to get you to safety in a pinch.