Instead of explaining with words she opens her mind just enough to let him get the concepts:
Expression, the effect of trying and succeeding to do magic, it's idiosyncratic and causes different effects depending on the kind of mage a person is.
Mana, the fuel every form of magic uses, personal and (normally) impossible to transfer, built up from birth.
Arcanism, (permanently, but personally) binding (in one sitting, you can't stop in the middle and continue later) magical effects (known as spells) to actions and words and symbols (collectively known as a spell's incantation) in order to project them or write them down so other people can do it. Expression tends to cause an unfortunate effect to be attached to an unfortunate incantation. Mana has no hard cap and starts charging up from birth, whenever the arcanist performs one of their spells' incantation while having enough mana the spell will be cast. Typically concerned with limited-duration or instantaneous effects, can also create scrolls that allow other people a single use of a spell.
Enchantment, (permanently) attaching (in one sitting, you can't stop in the middle and continue later) magical effects to inanimate objects (known as artefacts). Expressions tends to cause a random inanimate object nearby to become an artefact. Mana has no hard cap and starts charging up from birth; artefact creation costs mana, and every artefact has its own mana charge which runs down as it's used or as time passes and needs to be periodically recharged. Artefacts are automatically more durable than their mundane counterparts, and can store information or mana (which can then be drawn by other enchanters).
Elementalism, being able to apply magical effects (known as blessings) to oneself, elementalists have a (possibly infinite) list of magical effects to pick from, and what counts as a magical effect depends on the elementalist's specific mental architecture. Expression doesn't tend to have any extraneous side effects. Mana has a hard cap per blessing, all blessings start up charged with exactly the same amount of mana (although this is not known for sure by non-metamancers), mana only recharges while an elementalist has a blessing active but not being used (elementalists always have one and exactly one blessing active from when they Express and can only switch active blessings while not using them). Blessings are typically safe for their user (a blessing to go through solid objects for instance does not allow its user to suffocate while using it) unless the user decides to be reckless about it (stop flying while very high up).
Metamancy, being able to see and manipulate but not produce all magic. They can transfer and convert mana between people, they can unbind incantations from spells, they can turn an artefact into a mundane object, they can allow elementalists to recharge mana while not having a blessing active, they can nonconsensually steal mana from other mages and from artefacts, they can allow arcanists and enchanters to take breaks while developing spells/artefacts, they can use any mana they have stored to perform the effects of any of the other sorts of mages, they can lock up or steal a person's blessing. Expression has no external effects if no other mages are around, and accidental use of a mage's magic if they are. Universally reviled and considered sinful and tainted and evil.
A very small share of people have magic at all, and out of them not everyone manages to Express their magic (due to not finding the idiosyncratic mental action to do it), and out of those who do only a tiny fraction are metamancers.