This is the Accelerant version of character "hit points" or "health". When a character runs out of Vitality, they become at risk of dying. Vitality can be decreased by a number of in game actions or effects - being attacked, being damaged, use of certain spells, etc. Conversely, Vitality can be restored by various spells, abilities, skills, and effects.
Not all negative effects in game result in the loss of Vitality - for example an ability that can cause you pain may not deal any Vitality damage.
The amount of Vitality a character or creature has, and what determines that amount, can vary based on the campaign setting. Often it is based on character Attributes.
The following passages contain transcripts from the Accelerant core rules book:
Vitality is a count of how much damage you can take before you collapse. As you take damage, your Vitality points are exhausted.
Vitality points can never drop below zero. Healing refreshes Vitality, and these points are also refreshed at the beginning of each event. If your Vitality ever reaches zero you will collapse and become Unconscious. Unconscious characters that are taken down by uncalled melee or missile hits are Stable. Characters taken down by any other kind of damage, from called melee or missile hits, from firearms, from traps, from packets, or from anything with a Verbal become Unstable.
If you fall unconscious but you are stable, you will remain unconscious for five minutes. After that time you will wake up with 1 point of Vitality. If someone hits you with an attack for called damage while you are unconscious and stable you will become unstable and begin your 1 minute count. Other effects can be inflicted upon you while you are unconscious and stable and those effects will still be active when you wake up. If healing raises your Vitality above 0 then you will become conscious. A Death Strike successfully delivered to your torso will kill you. If you are unconscious and you are unstable you are dying from blood loss and shock. You will linger for one minute before dying. A Stabilize effect will change your condition to stable and you will begin your 5 minute count. Other effects can be inflicted upon you while you are unstable and those effects will still be active if you wake up. If healing raises your Vitality above 0 then you will become conscious. A death strike successfully delivered to your torso will kill you. If someone begins to use First Aid on you, your count will be suspended until they stop the First Aid. If they call Stabilize you become stable and start your five minute count. If they do not finish the First Aid, your one minute death count will continue where it was before they started using the Skill.
When you die, all active effects on your person end unless an Imbue or Inflict effect specifically says otherwise on the Effect Card. Your remains will linger for five minutes before you change to a Spirit of the Dead and begin to walk to the Gate of Death. Only effects with "to Spirit" or with "to Dead" traits will affect a spirit of the dead.