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.
In the Storyteller System, Gnosis is ranked with dots, as well as boxes. The dots are permanent Gnosis while the boxes represent points of temporary Gnosis. A Garou character may not have more temporary points of Gnosis than permanent dots of Gnosis.
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.
Gnosis may be recovered through meditation, or through bargaining with spirits to have them share some of their Gnosis.
Gnosis may not be spent in the same round as Rage, unless a Gift or other effect specifically requires the spending of both points to activate.
In game, Homid characters start out with the least Gnosis (1), Lupus Characters with the most (5) and Metis characters start out in the middle (3). Kinfolk generally do not have Gnosis, but a special Merit will allow the Kinfolk to do so: 5 points in the Merit is worth 1 Gnosis, 6 points is worth 2, and 7 is worth 3.
A Kinfolk with Gnosis is immune from receiving the Embrace and turning into a vampire. They simply die.