It's basically a starter element. As in, the most basic of all elements... it's also non-elemental in that it can't be resisted, and nobody's actually weak against it.

Ether itself is kinda just magic essence, unformed in this state, and invisible to all but whomever's manipulating it at the time... anyways, usually when a character casts a spell, they change the ether they're using to fit whatever element and spell they're going to actually use, that is, unless they're using an ether spell. 

As far as mechanics go, it's a means for mages to have spells that can still do damage even if the enemy has a resistance or immunity to all of their available spells. Also, Sumable, since Ether's kind of a non-element, you can't really have an elemental of that element, sorry.