Light is one of the two infallable elements, alongside Darkness.
Light refers to elemental phenomenon that asserts power, causes bodily harm or alerts to danger. It's associated with angels and Arcana, who were given Light in case the three kingdoms needed to be defended from Puritanical threats such as that of the Horsemen. However, angels were instructed by Them not to use Light without caution, as Light, unlike Darkness, can be used to kill, or at least cause harm. This is part of why angels (with the exception of the Horsemen-slayer Cyna) were designed to think more existentially and were given a more fragile mental fabric than the demons of Nocturna (who can only wield Darkness) since more was at stake if an angel were to not consider and agonize over all the implications of their actions, and because demons needed to be caretakers for the dead without burning out, before the Unrapure combined the three realms into one.
When Light is untillized as Magick, it is called Pyromancy.