It's Writer's Wednesday and the topic is backstory.
A character's backstory can be approached from two perspectives.
From a writer's point of view, backstory is the information the author must have to create a character's voice and motivate the actions, thoughts and decisions made throughout the story. An editor once told me that if someone asked what my character ate for breakfast or watched on TV, I should have an immediate answer regardless of who, what, where or when my character existed. The point the editor was trying to make was that a writer should know their characters on a much deeper level than might appear necessary to the plot. That knowledge will help answer the all important "why." Why did the character do or say what they did at any given moment? Because the author needed them to for that plot point is not the answer you're looking for.
From a reader's point of view, backstory is the information the reader needs in order to believe and accept a character's behavior. That backstory will consist of events and experiences that occurred before the opening page of the book. The challenge for the writer is to determine where and how to convey that information. One of the most common mistakes is to give too much too soon. Another hazard is to fall into a rhythm of providing a bit of backstory just in time to explain the next action...a pattern that will come across as unnatural or contrived. Let the readers become invested in the characters and their struggles then weave the backstory through the book providing just enough to enable the readers to draw their own conclusions about the "why" behind a character's choices.