The yellow bar is the armor.
Quote from The Forest Wiki:
The player can make armor by using the Lizard Skin. The yellow meter on the left of the health bar indicates the amount of armor you are wearing. The armor can be destroyed by fire, and enemies.
[Edited by dragonmaster505, 6/2/2014 4:22:36 PM]
[Edited by dragonmaster505, 6/2/2014 4:22:57 PM]