Inner Power is a magic user's total magical energy at any given time. In most cases, this is simply potentia within a person's olum and in a nearby leyline, but in cases of willpower burn associated with usage of will magic, it is instead a person's own willpower that is consumed. The consequences of this are described in greater detail in the will magic article.
Inner power is in essence a magic user's total ability to use magic at any given time. It is the sum of a person's current leyline magic (environmental potentia), ability to utilize knowledge magic (resulting in improvement of potentia efficiency), and willpower (direct burn of internal potentia).
A magic user seeking greater mastery shouldn't neglect any of the three. Through utilizing leyline magic more heavily, one's ability to control their own flow of magic is improved. Through utilizing knowledge magic, one's ability to continue to use magic further from a leyline, and their endurance, will improve. Through utilizing will magic, one's total internal potentia capacity will improve.
Every magic user will have a different level of proficiency with each of the three branches of magic, but each magic user would benefit by utilizing all three branches in some reasonable balance.
It may be helpful to think of a magic user as a hybrid car, with potentia being battery power, and willpower being gas power. A leyline is analogous to a charging road which passively charges the hybrid car as it drives over, and in this way, a car on a charging road would be able to drive so long as the road is powered, without ever needing to activate its engine. Knowledge magic is analogous, in its usage, to regenerative braking. It may refund some of the used potentia through skillful application, but just as regenerative braking, it cannot restore more energy than is used. There is no kinetic energy to be regained from a car that is stopped. A magic scroll may restore some potentia, but never more than was consumed in its creation.
Willpower burn is like continuing to drive the car after its batteries have run completely dry, and the road is no longer a charging road. In mild cases, where the user has not driven very far, the user can simply return to a charging road before running out of gas and, from there, find a gas station to refuel. But in extreme cases where the user has ventured too far away from charging stations, the user may become stranded once they run out of gas.
This is where the analogy breaks down. In the hybrid car analogy, a stranded car may be rescued by another person, and then towed back to a gas station or charging station. But with willpower burn, a person cannot be rescued if they burn too much. They will be permanently injured, and will develop various physical and mental disabilities as a result.