Player Plot Armor 5x HP
Raises Player Unit health by a factor of 5x at all difficulty levels.
This means
Easy: 10x base health
Normal, Hard, and Heroic: 5x base health
In essence, 10x normal health means taking 1/10th the damage you should take per hit and for 5x, 1/5th the damage.
This does not affect AI units (including allied units under AI control.