I’ve seen quite a bit of confusion surrounding this game, even two decades later. My friends were guilty of wrong-think much later than I can forgive. Here’s a simple take without all the deets:

In the world of Spira, summoners call upon fayth - people who willing sacrificed themselves - to bring forth aeons and combat the forces of Sin, a giant beast that attacks large groups of people and machines.

Tidus, star blitzballer(and player character) from the technologically advanced Zanarkand, is seemingly pulled 1000 years in the future to find his city is gone and Sin’s influence has made technology taboo under the theocracy Yevon.

As you join the summoner Yuna and her guardians in her pilgrimage to defeat Sin, it is revealed that your 'dead' father Jecht was also pulled into this world and served as guardian to Yuna's father(who previously defeated Sin alongside Tidus' mentor Auron.)

Tidus is devastated when he finds out the final summoning to defeat Sin results in the death of the summoner, as he and Yuna have grown close. Additionally, it turns out a guardian must sacrifice themselves to become the final aeon and will become the next Sin, which was Jecht’s eventual fate.

Tidus is desperate to find a way to stop Yuna's seemingly inevitable sacrifice as they near the end of the pilgrimage and cross the restricted Mount Gagazet, where huge amounts of fayth are entombed, actively assisting in summoning. Falling into a dreamlike state, the fayth communicate to Tidus and explain why he was ripped from his home.

In a last ditch attempt to save Zanarkand from destruction in an ancient war, the remaining population became fayth to summon a memory of the city as it was and remain forever hidden. You are not from the past, rather a 'dream' made by Yu-yevon, the being who eternally summons the city and it's inhabitants, along with Sin to protect it.

The fayth are tired of dreaming and ask you to defeat Yu-yevon and end the cycle of Sin, which will of course result in YOUR end rather than Yuna's.

This is the last time you fight together.

Proper nouns are bold, in-universe terms are in italics. But you figured that out already didn’t you?