Backstory is the answer to how the character reacts.
It explains how and why and who they are. Write up a history, even if you don’t use in the story. In particular, write about secrets as it is these that explain the character’s decisions, helping to dramatise and motivate the turning-points.
The question to ask is ‘What does this information do to the character now?’ If it doesn’t affect the action, then leave it out.