The Al'Noth is what spellweavers call the magical energy that makes spells happen. The energy of the Al'Noth is channeled, or woven into patterns that in the end cause certain effects. These are commonly known as spells.
Spellweavers are able to see this energy as they weave it, either by being taught or by having a knack for it.
Most people believe that the magical energy of the Al'Noth is made available to all by the goddess Lucinda. Where she gets the energy from is unknown, but subject to a lot of theorizing.
Some say she is the embodiment of the Al'Noth and that the weaving of a spell is sharing the essence of Lucinda's being. Others think this is a step to far and do not consider Lucinda and the Al'Noth to be the same thing, or even partially overlapping.
A small group claim that the Al'Noth just is, and that even without a deity of magic it would still be there. This is of course considered heresy by the Lucindites who claim that without their goddess, there would not be any magic.
Note: for the purposes of NWN, both sorcerors and wizards are classified as spellweavers. Wizards are those who have been taught to use the Al'Noth, sorcerors have the knack.