Mana is the magical energy used to power spells and enchantments.
All mana ultimately comes from the Ethereal Plane. Any world cut off from the Ethereal will be unable to naturally replenish any ambient mana that is used. Catalysts are capable of drawing mana directly from the Ethereal, bypassing otherwise mana-dead worlds. It is easier to use mana at a node, where the Veil is thinner, as well as anywhere inside the Ethereal Plane itself, such as on any plane or in a pocket-world.
Mana resonates with the magical signature of the person who originally channeled it and filtered it through their soul. Any skilled Seeker can identify who cast a spell if they know someone's magical signature.