Gnosis

Gnosis is similar to the Polynesian concept of mana, or the Eastern concept of chi. While all creatures have some spiritual component, Garou can consciously control and use it. Garou are quite literally half-spirit already and Gnosis reflects how connected they are to their other half. Gnosis can't be precisely measured, but if you think of it like water, it makes comprehending the difference between a Garou and a human easier. A human has a cupful to last a lifetime, a Garou has access to an endlessly replenishing river.

Gnosis can be used to bribe spirits, activate fetishes (magical artifacts) and Gifts, and to enter the Umbra. Creatures without enough Gnosis generally cannot perceive or enter the Umbra.

Gnosis is a renewable, but dwindling, resource. Garou can regain it at sacred sites, by meditating, or bargaining with spirits. Sacred sites provide Gnosis, but if they become defiled, it can corrupt the Garou, or there simply may be no more available. The water analogy works well here. They need water, but they need to protect their sources because drinking tainted water may make them sick, and if they aren't careful, their well may go dry.