Rochester tells us Bertha’s backstory (indicated here in italics):
Bertha is a young debutante in Jamaica, where all the boys love her and her beauty and musical talent make her extremely desirable.
Bertha’s family and Edward Rochester’s family arrange for their marriage.
Bertha is mentally unstable and (apparently) unhappy, and she behaves erratically, screaming at servants, drinking a lot, and sleeping around.
Bertha is diagnosed as insane and kept in a locked room.
After four years of marriage, Rochester takes Bertha back to England and confines her at Thornfield with Grace Poole as her nurse and jailer.
Bertha remains at Thornfield tended only by Grace for ten years.
Rochester comes back to Thornfield, bringing his ward Adèle and a governess, Jane Eyre.
Bertha becomes aware of Rochester’s attachment to Jane.
Bertha sneaks past a drunk Grace Poole and sets fire to Rochester’s bed in the middle of the night.
Bertha’s brother shows up to visit her; he tries to talk to her alone, but she stabs and bites him.
Bertha sneaks out of her room again and goes to Jane’s bedroom. Bertha examines Jane’s wedding dress and honeymoon luggage and tears her veil in half.
Rochester shows Bertha to Jane, a lawyer, and Bertha’s brother and describes her condition. Bertha behaves like a wild animal and tries to bite Rochester’s face.
Bertha sets fire to Jane’s old bed in the middle of the night and burns Thornfield down. As it’s burning, she climbs up onto a high wall and jumps off before Rochester can stop her, effectively committing suicide.