She was promised to Gerald after he did something for her mother. She was trained as a Witcher and later as a mage after Nilfgaard annihilated her kingdom and she had to flee.
She went wandering various dimensions, even landing in King Arthur's "era" at a point trying to flee Nilfgaard who were trying to marry her to their Emperor because she's the last legal heir to her small kingdom. Turns out he's her dad, by the way
Oh, and Ciri had a lesbian relationship with a paedophile female bandit for a few years. No. Joke.
Said invasion kicked off the war were... the redhead female mage, forgot the name sorry, got her scars and "died" and all that. Brenna and so on where the follow up war, still connected to that. The new war in the end of Witcher 2 is effectively a result of that as well.
In the end she returned from the various dimensions, travelled with Jeneffer to the city were Gerald got killed by the stable boy and then transports them both to "Albion" where Gerald heals.
Oh, she also visited the dimension the Wild Hunt comes from.