Mana was a form of energy comprised of countless tiny particles as small as a photon, yet far slower because they have mass. But they had the unique trait of being unable to interact with the fabric of space in their raw form. This means that while mana has mass, it isn't affected by gravity and is a great source of energy.
When a wizard casts magic, they either use the mana stored inside their bodies, the outside mana, or a combination of both, and then force those particles to either temporarily take the form of regular matter or push regular matter to create spells.
For example, using those particles to push electrons around in the air creates lightning, and using that lightning, a wizard can break down the moisture in the air into oxygen and hydrogen and then burn that to cast fireballs. It was also possible to directly use those particles to agitate matter in the air to heat up and ignite flame.
