A spell key is one of the little tricks that canny wizards pick up to allow them to bend the rules that the planes have set on magic. On many planes, it ain't possible to cast spells from certain schools, or else the effects of those schools are altered or diminished. Spell keys can be general (which means they affect a whole school on a given plane) or specific (which means they affect only one spell on a given plane). The exact nature of spell keys depends on the plane and spell in question; it's usually a symbolic gesture to the Laws of Magic. Whatever it is, using a spell key'll add 1 to the casting time of a wizard spell. It'd also be best to bear in mind that not every spell has a key: for example, wild magic'll never work on Mechanus, whatever the key, as its nature is just too antithecal to the plane.