The Weave is the interface layer between the Aether below and the Enneract above. A Weave is naturally formed when a cumulative mass of information collects at a point on the Enneract, forcing a connection to the underlying Aetheric field. Most localities have the requisite mass to form a weave, and there are recorded events of individuals successfully accumulating enough to artificially generate one.
As an interface, the weave allows beings to safely draw energy from Aether without exposing themselves to the dangerous undercurrent. The techniques used to draw from a weave are specific to that weave, so a mage from one locality will likely be unable to cast magic in another without great effort and preparation.