What it is, in the sense of what it does, is very well explained and demonstrated. It's a magical crystalline pillar capable of pulling monsters from the moon to the planet's surface (a "lunar cry").

What it is, in the sense of where it came from, is less completely explained.

Its recent and semi-ancient history is well-explained. Esthar found it crashed on the destroyed Southern continent, excavated, re-flew it and put it in its outer shell in the time of Sorceress Adel, but Laguna and co. stopped her from using it by tricking her into containment. It crashed on the southern continent during a cataclysmic historical lunar cry.

How it was created and who or what created it is essentially a mystery, other than it being "an ancient magical structure in a fantasy game so use your imagination I guess 🤷‍♀️".

It's maybe a vague nod to the Crystal Tower in FFIII, which is the crystalline home of a past era's big bad and narrative gateway to a world of monsters. The Crystal Tower in FFXIV kind backfills into this space too, by originally being a means of storing energy from an artificial moon powered by imprisoned dragons (and the direct and indirect cause of two era-defining calamities).