The Life globe on the lower-left screen represents the amount of health your character has based on the overall Vitality stat from your character base stats and extra permanent Vitality points acquired through weapons, armor gear, and + vitality runes.
A character loses Life through being hit by melee, ranged and magical attacks, but can also be affected by traps and objects bound to the Physix engine behaviour.
Life can be replenished with Health Globes that randomly drop when monsters are killed. You just need to walk over these globes to replenish life over time. Other means are Healing Potions, Elixirs, rejuvenation potions, shrines, and magical items with + replenish life.