Magic is pervasive, but usually fairly low level.
Magic "flows" like an insubstantial aether with pockets of higher or lower magic moving like invisible clouds meaning that even if the magic power is strong one moment, it might change back to normal again moments later.
Magic tends to flow from higher magic areas to lower ones, but will always "collect" near places of beauty or other locations that would attract fairies.
Mages are more tightly bound to magic so can often get a rough sense of the general magic level around them. Non-mages can sometimes get a more abstract "feeling" for the magic level, like feeling more positive when a higher level pocket moves through the area.
Effects of Magic Levels
Very Low Magic Level
Enchantments "recharge" like a battery by absorbing magic from the area. So an enchantment in a very low magic area will still have the same effect, but it will take much longer to recharge.
For a mage's spells, they will take much longer to prepare and will be slower to cast, but will have the same general effect as "normal".
Some spells are more sensitive to a lower magic level. These spells can not be prepared in a very low magic level. Also, they will be less effective, and the prepared tokens or enchantments will drain and become useless in a short time.
Normal Magic Areas
Magic is normally low level and pervasive.
Higher Magic Areas
In higher magic areas, spells and enchantments are more effective. Casted lights will be brighter or last longer. Healing spells will be more effective. Combat spells will cause more damage. Enchantments will recharge at a higher rate, and tokens will be easier to prepare.
Higher magic levels might be called "normal" in other worlds.
Elves tend to live in higher magic areas.
Very High Magic
Very High Magic areas are very rare outside of a Fairy Storm and can be fairly dangerous.
In a very high magic area, prepared tokens can go off randomly or catastrophically or even cause the wrong (but related) effect: a light spell might cast lightning or fire, or a healing spell might cause the patient's skin to grow thicker or even the patient to grow a new appendage.
Enchanted items will often activate without warning and the bound spell can be damaged from absorbing too much magic.
Mages have to be very careful in very high magic areas lest their casual thoughts be cast as random spells.
Animals trapped in very high magic areas can be mutated into their magic-bound cousins. Horses will become pegasus, lions become manticores. This usually does not turn out well for any humans or non-magic creatures in the vicinity.
See also Fairy Storms.
Changing the Magic Level
When a spell is cast, it "pulls" from the local magic field. Different spells can pull more or less or from a larger or smaller area. The spell-caster's skill will also have some effect on the range of magic impacted. One or two spells will have little effect, extra magic will flow in from other areas and restore the status quo.
If enough spells or a few powerful spells are cast, it can drain the local magic level causing subsequent spells to be less effective.
If a mage uses too many spells in an area in too short a time, they can deplete all the available Magic in an area.
Depletion can also rarely happen as a natural phenomena as the magic aether just leaks out of an area.
The area will be barren for some time until the Magic slowly re-fills the area from Fairy activity or by seeping in from nearby wells. Sometimes a "dead zone" will drift around, almost like a cold breeze, as the Magic shifts to fill the void.
In a dead zone, enchantments will usually fail or flicker and new spells will fizzle or completely fail to go off. Tokens and temporary runes will more quickly discharge into the local environment.
Someone entering an area that has had its Magic depleted will often feel something akin to a "spiritual chill". Others have described it like "someone walking over your grave".
Mages can always can immediately sense a pocket of depleted Magic.