Every character is entitled to one “free” death.
If a PC who has a Free Death left dies in battle the DM will figure out a way for that PC to survive in some manner: through divine intervention, the sudden appearance of a sympathetic friend who happens to know the True Resurrection spell, or the like. DMs are free to be creative on this front.
Once a character has used their Free Death the normal rules for Death and Resurrection apply. The DM will also ask the player to make a note on their PC’s Adventure Record that said PC’s free death has been exhausted.