A caern is a natural upwelling of Gnosis, the spiritual energies of Gaia, used by the Garou to power their gifts.
Caerns also have a very thin gauntlet, allowing easier congress between the physical world and the Umbra. It's possible that a caern's Gnosis actually comes from the Umbra in this fashion.
Caerns are very holy sites to the Garou and other Fera, and they will work to their utmost to claim them and protect them. Occasionally this will mean claiming it from another group who isn't looking after it responsibly.
The Garou social unit of the sept tends to be centered around a caern. Moon Bridges may be opened allowing rapid travel between certain allied caerns.
The caerns of the Black Spiral Dancers are referred to as Pits.
Each Caern, that is active, is protected by a Garou Sept .