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Depends on which fate you're watching tbh since there's so many shows, games, and manga. The basic premise for most of them, including the Og stay night, is that there's a holy grail war in which mages from curtain bloodlines battle it out for the holy grail which grants the winner a wish. the mages battle each other using "servants" which are the reincarnations of historical or fictional heroes.
That's the super simplified version but there's a shit ton more lore about things like akasha, the earlier grail wars, why the grail was made, the difference of magecraft and actual magic and why mages want it, etc..