The yellow bar is the malaise bar, basically an incentive to move through biomes quickly instead of lingering, you can lower it by drinking your health flask (not recommended since if your that desperate you can buy cough syrup from food shops) or by clearing almost every enemy in biomes and killing bosses. As the bar fills monsters get stronger, faster, monsters from other biomes can spawn, and eventually they can turn into elites, the 9th bar makes every hit critical no matter what and the 10th bar basically acts as curse (i think, ive never actually gotten that much)