Mana is one of the major sources of magic in Odiem, the other being the power of the deities. It is a physical concept, largely made up of a crystalized liquid, although it can exist in solid form as well.
It is the result of all biological interactions between living species, as a sort of byproduct. It was formed thousands of years before the Primordial Age, and has since crystalized underneath the surface of Odiem.
Mana begins its life as an invisible, microscopic crystal that floats in the air. These crystals are harmless to the health, and are always very sparsely distributed in the atmosphere, meaning that the chances of actually breathing one is very low.
Similar to how carbon dioxyde is a byprodut of respiration, Mana Crystals are formed as a byproduct of emotions that two living beings can have for each other, such as love or hate. Every time a living creature loves or hates another, its body will produce a few crystals as a result of its brain producing certain chemicals.
Over time, these crystals deposited themselves into the earth, and accumulated for thousands of years until they formed rivers and large crystals. The vast majority of these rivers are found underground, although there is a massive mana river known as the Mana Spool within the island of Fiyo Thalore. The Mana Spool is 573 kilometers long, and it's the largest exposed vein of mana in Odiem.
The beings that dwell in these rivers are known as Spirits, and they are made up of pure Mana. They are not as powerful as deities, but they are able to magically alter their surrounding and even teleport to other veins of Mana. They are also not able to produce offspring in the same way deities can, instead reproduing by simply appearing and disappearing into the Mana rivers.