Some mages are inborn to specific Talents or abilities. They require no training to use their particular specialization, and will often begin using it in early childhood without any idea what they're doing.
Inborn magic is instinctive and comes as easily to the mage as breathing. Inborn mages tend to be more powerful with the Talent they are inborn to than mages who were simply trained, but trained mages will generally wind up being more skilled and capable of using a Talent with greater control and precision. However, inborn mages can also be trained or practice on their own in order to gain the same skill if they're willing to work at it.
One peculiarity about inborn magic is that a mage's ability may not be one typically seen outside of non-inborn magic. It may be hyperspecialized, work strangely, or simply be something that has never been seen as a spell before. This is often how new magic is discovered.