An Interface Spell is a specialized spell that serves a singular purpose: interacting with a specific magic item. Such spells are part of the school of Metamagic. They have no effect when cast on anything but the enchanted object they were designed to interact with.
Complex enchantments that use the Interface component specify an interface spell as part of their metamagical code. If you know an enchantment with an interface spell, you automatically know the interface spell as well. This can serve as a useful security measure, as even other mages won't be able to operate an enchanted item unless they know the interface spell.
It's also possible to learn an interface spell separately from the enchantment it's based on. If you know the Interface component itself and can create a spell script, you can choose to create a script of the interface spell by itself. For magical artifacts that use interface spells, this is the only way the interface spell can be learned, as the enchantments aren't grouped into a single unit that can be learned.