I’d think it were just a gameplay element, except it does come up in dialogue sometimes, but not often enough that I’m really clear on what it’s supposed to be.
In terms of World of Warcraft, Mana seems to be a very generic form of mystical energy which can be transmuted into almost any other type of cosmic energy under the right circumstances. Priests use it to create Light and Shadow, Mages use it to create Arcane, Fire, or Frost, etc…
I’d say it’s like Water, and we just add our own flavor.