Yellow Bile is one of the four Humours. It is used in combat to perform combat actions. Every combat action drains your Yellow Bile. Yellow Bile will also slowly drain over time while you are lifting and carrying objects around.
In addition, your current level of Yellow Bile determines how much damage you do in combat. Since your Yellow Bile is drained by combat actions you do less damage the more combat actions you do.
Leveling Yellow Bile will also increase your carry weight limit before becoming encumbered.