It's negative energy damage
The ability is talking about negative energy damage, which is the opposite of positive energy damage, and is simply another damage type like Fire, Cold, Electricity or Acid. Each of these damage types originates from one of the elemental planes.
However, for negative energy, it comes from the Negative Energy Plane and will hurt living creatures. Its opposite, positive energy, from the Positive Energy Plane, will harm undead creatures and usually heal the living.
But there are several other types of energy damage. Damage types are explained on page 293 of the playtest rulebook describing Damage Types:
Damage comes in many different types. Weapons, spells, and abilities indicate the types of damage they deal, and immunities, resistances, and weaknesses function against certain damage types. An effect that deals a type of damage gains the matching trait, if appropriate.
Bludgeoning, piercing, and slashing damage typically comes from weapons; these types are collectively called physical damage. Spells, special weapons, and environmental dangers can cause many other types of damage. Acid, cold, electricity, fire, and sonic are all types of energy damage. Force damage comes from pure magical energy and can damage ethereal creatures, such as ghosts.
Negative damage harms the life force of living creatures, and positive damage hurts undead. Effects that exist purely in the target’s mind deal mental damage. Weapons and effects keyed to a particular alignment can deal chaotic, evil, good, or lawful damage.