Backstory explains how and why your character is the person she is: shaped by a background built on family, education and past experiences.
The most interesting backstories are the secrets revealed under the greatest stress when her morality, beliefs and values are tested to the extreme.
Never give the audience anything that they don’t need to know. Past secrets get revealed only through current action.
What backstory does the character keep a secret from others? Why? What would make her reveal this?