From Path of the Vampire Wiki
"Anima" is the Latin term for the animating principle. It can be translated as being "breath", "life" or "soul".
Anima is the energy that keeps all physical beings alive, and although humans or animals cannot use it in the same way that vampires do, it is still present in the all of them, until the day they die. Even after death, a human spirit will retain some anima, allowing them (sometimes) to return to the physical world as spirits and occasionally communicate with the living.
Vampires are dead, and so they do not naturally have anima (or at least, they have minimal amounts of it). To fuel their spirit-made bodies and stay in the physical realm, vampires require anima, which they extract from the blood of living beings. Each day a vampire remains well fed, with enough blood in their veins, they will gain extra anima. This is why a vampire drained of blood is in danger of dying; it is the anima in their blood that enables them to heal. Even with anima still left in them, they cannot use it to heal if they have no blood in their system. The exact link between anima and blood is not completely understood, but it would appear that blood stores anima and can be converted into it, but anima appears not to be able to be converted back into blood, unless the individual has developed certain necromantic or mystic talents.
Vampires with enough extraneous anima are able to use it to perform supernatural feats which would be impossible for a creature made entirely of flesh and blood. By draining the life from living creatures, they can use this extra energy to do things such as teleport, control fire, heal, manipulate the minds of other individuals and even shape shift.
When a vampire dies, they are able to grow strong enough to re-substantiate their semi-physical bodies (or possess new ones, depending on their path), if they drain sufficient amounts of anima from the life force of other spirits. Some vampires can even extract anima from spirits while they (the vampire) are still alive.
In a roleplay sense, anima exists as a concept which characters can talk about, but it is not measured in "anima points" in the same way we measure it out of character or in game. It is not measurable in the same way pints of blood are measurable, as there are no instruments available to a vampire which can tell them how many anima "points" they have remaining to them.