What in particular is confusing to you?

I personally thought enough was explained without reliance on knowledge FF12 and Tactics, though admittedly some of the explanations come from talking to NPCs between quests.

Dalmasca was conquered by the empire 30 years ago, and after Doma and Ala Mhigo were freed Dalmasca tried doing the same. In response the empire destroyed their capital.
We come in here, exploring the ruins of the capital and discovering that it was built on the ancient kingdom of Ivalice which was thought to be a myth.
Ivalice had a civil war called the War of the Lions, and during that war twelve heroes called the Zodiac Braves appeared to help a commoner man named Delita become king. But during the quests we also discover that they fought an alien monster named Ultima the High Seraph, and several of their group sacrificed themselves to seal Ultima away.
In the modern day Ultima is trying to free herself using the blood of the descendants of the leader of the Zodiac Braves, Ramza Beoulve. She sends out Auracite which mutate people into monsters called Lucavi.
The Lexantale family we are working with are descendants of the Beoulves and so Ultima uses Auracite to drag Alma and later Ramza Lexentale (named after his ancestor) to her prison so she can free herself. We kill Ultima and the spirits of the Zodiac Braves thank us for finally being free of their duty keeping her imprisoned.