You die when you have taken 6 Wounds (unless you have an ability that changes this number). There are ways to revive a hero who has died, but they are rare and often come at a very steep cost.
Death
Core Rules v2 p. 9
Official Variants
1Blaze of Glory
VariantCore Rules v2 p. 55
When a hero suffers their final Wound, they get 1 full turn to act before they finally die.
Related Rules
4ConditionsReference / Indexes
Hit Points, Dying & WoundsFundamentals / Health
Safe RestsAdventuring / Rest & Recovery
WoundsFundamentals / Health